rockchip: Fix MMC boot order

Basically all, i.e. rk3036.dtsi, rk3128.dtsi, rk3xxx.dtsi, rk322x.dtsi,
rk3288.dtsi, rk3308-u-boot.dtsi, rk3328-u-boot.dtsi, rk3399-u-boot.dtsi
and px30-u-boot.dtsi Rockchip SoC devicetrees which have mmc indexes
are defining eMMC as mmc0 and sdmmc as mmc1.
This means that the rule to try to boot from the SD card first is ignored,
which as per comment is what we want and is important for distros, which
rely on that.

Fix this by setting the correct mmc index, i.e. first from mmc1 (SD card),
second from mmc0 (eMMC).

Signed-off-by: Alex Bee <knaerzche@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>