Add Gigadevice SPI rom support

Add support for GigaDevice SPI ROMS.
The GD25Q64B device has been tested, the other rom devices added to the
file have not.

Change-Id: If35676ca6b90329f15667ebb32efa0d1a159ae91
Signed-off-by: Martin Roth <martin@se-eng.com>
Reviewed-on: http://review.coreboot.org/1747
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/drivers/spi/spi_flash.c

@@ -218,6 +218,9 @@ static struct {
 #if CONFIG_SPI_FLASH_EON
 	{ 0, 0x1c, spi_flash_probe_eon, },
 #endif
+#if CONFIG_SPI_FLASH_GIGADEVICE
+	{ 0, 0xc8, spi_flash_probe_gigadevice, },
+#endif
 #if CONFIG_SPI_FLASH_MACRONIX
 	{ 0, 0xc2, spi_flash_probe_macronix, },
 #endif