sb600spi: Add spireadmode

Added spireadmode for >= Bolton.
Do not override speed or read mode for >= Bolton if parameter not
specified.
Minor cleanup of sb600spi.c code.

TEST=Manual: deploy on tremblye read flash using various parameters
BUG=b:147665085,b:147666328
BRANCH=master

Change-Id: Id7fec7eb87ff811148217dc56a86dca3fef122ff
Signed-off-by: Rob Barnes <robbarnes@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/38833
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

flashrom.8.tmpl

@@ -497,11 +497,29 @@ Support of individual frequencies depends on the generation of the chipset:
 .sp
 * SB6xx, SB7xx, SP5xxx: from 16.5 MHz up to and including 33 MHz
 .sp
+-The default is to use 16.5 MHz and disable Fast Reads.
+.sp
 * SB8xx, SB9xx, Hudson: from 16.5 MHz up to and including 66 MHz
 .sp
+-The default is to use 16.5 MHz and disable Fast Reads.
+.sp
 * Yangtze (with SPI 100 engine as found in Kabini and Tamesh): all of them
 .sp
-The default is to use 16.5 MHz and disable Fast Reads.
+-The default is to use the frequency that is currently configured.
+.sp
+An optional
+.B spireadmode
+parameter specifies the read mode of the SPI bus where applicable (Bolton or later).
+Syntax is
+.sp
+.B "  flashrom \-p internal:spireadmode=mode"
+.sp
+where
+.B mode
+can be
+.BR "'Normal\ (up\ to\ 33 MHz)'" ", " "'Normal\ (up\ to\ 66 MHz)'" ", " "'Dual\ IO\ (1-1-2)'" ", " "'Quad\ IO\ (1-1-4)'" ", " "'Dual\ IO\ (1-2-2)'" ", " "'Quad\ IO\ (1-4-4)'" ", or " "'Fast\ Read'" "."
+.sp
+The default is to use the read mode that is currently configured.
 .TP
 .B Intel chipsets
 .sp