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>