drivers/spi: provide a mechanism to obtain the SPI flash boot device

The MRC cache wants to be able to access the SPI flash boot device.
Allow an easy way to provide that so that there isn't duplicate
spi_flash objects representing the same device.

BUG=chrome-os-partner:56151

Change-Id: Iba92e8bb8a6060cdd327b10f5f8ec23ac61101e7
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/17715
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/drivers/spi/boot_device_rw_nommap.c

@@ -95,3 +95,9 @@ const struct region_device *boot_device_rw(void)
 
 	return &spi_rw;
 }
+
+const struct spi_flash *boot_device_spi_flash(void)
+{
+	boot_device_rw_init();
+	return car_get_var(sfg);
+}