arm64: dts: rockchip: Assign a fixed index to mmc devices on rk3399 boards.

Recently introduced async probe on mmc devices can shuffle block IDs.
Pin them to fixed values to ease booting in environments where UUIDs
are not practical. Use newly introduced aliases for mmcblk devices from [1].

[1]
https://patchwork.kernel.org/patch/11747669/Signed-off-by: Markus Reichl <m.reichl@fivetechno.de>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20201104162356.1251-1-m.reichl@fivetechno.deSigned-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3399.dtsi

@@ -29,6 +29,9 @@ aliases {
 		i2c6 = &i2c6;
 		i2c7 = &i2c7;
 		i2c8 = &i2c8;
+		mmc0 = &sdio0;
+		mmc1 = &sdmmc;
+		mmc2 = &sdhci;
 		serial0 = &uart0;
 		serial1 = &uart1;
 		serial2 = &uart2;