spi25: Use common code for nbyte read/write and block erase

Introduce spi_prepare_address() and spi_write_cmd() and use them in
nbyte_program, nbyte_read and block-erase procedures. The former
abstracts over the address part of a SPI command to make it exten-
sible for 4-byte adressing. spi_write_cmd() implements a WREN + write
operation with address and optionally up to 256 bytes of data. It
provides a common path to reduce overall redundancy.

Also, reduce the polling delay in spi_block_erase_c4() from 500s to
500ms as the comment suggests.

Change-Id: Ibc1ae48acbfbd427a30bcd64bdc080dc3dc20503
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/22383
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>

spi.h

@@ -24,6 +24,8 @@
  * Contains the generic SPI headers
  */
 
+#define JEDEC_MAX_ADDR_LEN	0x04
+
 /* Read Electronic ID */
 #define JEDEC_RDID		0x9f
 #define JEDEC_RDID_OUTSIZE	0x01