drivers/spi/cbfs_spi: Provide implementation of boot_device_spi_flash

This allows callers to retrieve handle to the boot device spi_flash structure.

BUG=b:38330715

Change-Id: I1c07327115e0449cbd84d163218da76a6fa2cea0
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/19726
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/drivers/spi/cbfs_spi.c

@@ -134,3 +134,9 @@ const struct region_device *boot_device_rw(void)
 {
 	return boot_device_ro();
 }
+
+const struct spi_flash *boot_device_spi_flash(void)
+{
+	boot_device_init();
+	return spi_flash_info;
+}