uboot-imx

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>