rockchip: Allow booting from SPI

The u-boot,spl-boot-device property only allows MMC at present. Add SPI as
well for boards that boot from SPI flash.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

arch/arm/mach-rockchip/spl-boot-order.c

@@ -61,6 +61,9 @@ static int spl_node_to_boot_device(int node)
 		default:
 			return -ENOSYS;
 		}
+	} else if (!uclass_get_device_by_of_offset(UCLASS_SPI_FLASH, node,
+		&parent)) {
+		return BOOT_DEVICE_SPI;
 	}
 
 	/*