doc: Migrate doc about raspberry pi

Old doc:
https://wiki.flashrom.org/RaspberryPi

Change-Id: Ie3b7f2ed31f0b756815de055a2c58602263dac66
Signed-off-by: Anastasia Klimchuk <aklm@flashrom.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/86312
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

doc/user_docs/index.rst

@@ -12,6 +12,7 @@ Users documentation
     misc_intel
     in_system
     msi_jspi1
+    raspberry_pi
     misc_notes
 
 .. Keep misc notes last

doc/user_docs/raspberry_pi.rst

@@ -0,0 +1,56 @@
+===========
+RaspberryPi
+===========
+
+`RaspberryPi <http://www.raspberrypi.org/faqs>`_ is a cheap single-board computer developed in the UK
+by the Raspberry Pi Foundation with the intention of stimulating the teaching of basic computer science in schools.
+It can run a fully-functional GNU/Linux distribution and exposes SPI, I2C and several GPIOs on its expansion header.
+
+Prerequisites
+=============
+
+Use latest Raspbian (or any other distribution with a recent kernel).
+Run the following commands (or make sure these kernel modules are loaded successfully)::
+
+    modprobe spi_bcm2835 # If that fails you may wanna try the older spi_bcm2708 module instead
+    modprobe spidev
+
+Connecting the flash chip
+=========================
+
+To learn more about the RPi's expansion header refer to http://elinux.org/Rpi_Low-level_peripherals .
+If you use one of the first A or B models, please make sure to not draw more than 50mA from the 3.3V pin.
+Official numbers for newer models are hard to find but they use a switching regulator
+that should provide a much higher margin. If the flash chip is still placed in a foreign circuit
+(e.g. soldered to a PC mainboard) please refer to :doc:`/user_docs/in_system` for further details.
+
+========== =======================
+RPi header SPI flash
+========== =======================
+25	   GND
+24	   /CS
+23	   SCK
+21	   DO
+19	   DI
+17	   VCC 3.3V (+ /HOLD, /WP)
+========== =======================
+
+.. container:: danger, admonition
+
+   Always connect all input pins of integrated circuits (not only flash chips).
+
+In general the other pins (usually pin 3 is /WP and pin 7 is /HOLD) should be connected to Vcc
+unless they are required to be floating or connected to GND (both extremely uncommon for SPI flash chips).
+Please consult the datasheet for the flash chip in question.
+
+If your flash chip is detected but your read/write/verify operations tend to fail, try to add decoupling capacitors (one 100nF and one 4.7uF ceramic capacitor is preferred) close to the flash chip's power pin.
+
+Running flashrom
+================
+
+Flashrom uses the Linux-native SPI driver, which is implemented by flashrom's linux_spi programmer.
+To use the RaspberryPi with flashrom, you have to specify that programmer. You should always tell
+it at what speed the SPI bus should run; you specify that with the spispeed parameter (given in kHz).
+You also have to specify the Linux SPI device, e.g.::
+
+    flashrom -p linux_spi:dev=/dev/spidev0.0,spispeed=1000