drivers/spi: Add BOOT_DEVICE_SPI_FLASH_NO_EARLY_WRITES config

Added new config BOOT_DEVICE_SPI_FLASH_NO_EARLY_WRITES to accomodate
older x86 platforms that don't allow writing to SPI flash when early
stages are running XIP from flash.  If
BOOT_DEVICE_SPI_FLASH_NO_EARLY_WRITES is not selected,
BOOT_DEVICE_SPI_FLASH_RW_NOMMAP_EARLY will get auto-selected if
BOOT_DEVICE_SPI_FLASH_RW_NOMMAP=y.  This allows for current platforms
that write to flash in the earlier stages, assuming that they have
that capability.

BUG=b:150502246
BRANCH=None

TEST=diff the coreboot.rom files resulting from running
     ./util/abuild/abuild -p none -t GOOGLE_NAMI -x -a --timeless
     with and without this change to make sure that there was no
     difference.  Also did this for GOOGLE_CANDY board, which is
     baytrail based (and has BOOT_DEVICE_SPI_FLASH_NO_EARLY_WRITES
     enabled).

Change-Id: I3aef8be702f55873233610b8e20d0662aa951ca7
Signed-off-by: Shelley Chen <shchen@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45740
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/northbridge/intel/x4x/Kconfig

@@ -12,6 +12,7 @@ config NORTHBRIDGE_SPECIFIC_OPTIONS # dummy
 	select INTEL_GMA_ACPI
 	select CACHE_MRC_SETTINGS
 	select PARALLEL_MP
+	select BOOT_DEVICE_SPI_FLASH_NO_EARLY_WRITES
 
 config CBFS_SIZE
 	hex