rockchip: rk3308: add support for sdmmc boot

Some ROCK Pi S SKU/models are not equipped with SD-NAND (eMMC),
therefore SPL needs access to sdmmc: add it to rk3308-u-boot.dtsi
with bootph-all property.

Signed-off-by: Massimo Pegorer <massimo.pegorer@vimar.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/dts/rk3308-u-boot.dtsi b/arch/arm/dts/rk3308-u-boot.dtsi
index c8451b2..db2c20a 100644
--- a/arch/arm/dts/rk3308-u-boot.dtsi
+++ b/arch/arm/dts/rk3308-u-boot.dtsi
@@ -26,6 +26,11 @@
 	bootph-all;
 };
 
+&sdmmc {
+	bootph-all;
+	u-boot,spl-fifo-mode;
+};
+
 &grf {
 	bootph-all;
 };