cfi_flash: Support buffered writes on non-standard Spansion NOR flash

Some NOR flash chip from Spansion, for example, the s29ws-n MirrorBit
series require different addresses for buffered write commands. Define a
configuration option to support buffered writes on those chips. A more
elegant solution would be to automatically detect those chips by parsing
their CFI records, but that would require introduction of a fixup table
into the cfi_flash driver.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
diff --git a/README b/README
index 5d059e7..b29a0d2 100644
--- a/README
+++ b/README
@@ -2040,6 +2040,13 @@
 		This option also enables the building of the cfi_flash driver
 		in the drivers directory
 
+- CFG_FLASH_USE_BUFFER_WRITE
+		Use buffered writes to flash.
+
+- CONFIG_FLASH_SPANSION_S29WS_N
+		s29ws-n MirrorBit flash has non-standard addresses for buffered
+		write commands.
+
 - CFG_FLASH_QUIET_TEST
 		If this option is defined, the common CFI flash doesn't
 		print it's warning upon not recognized FLASH banks. This