ARM: dts: rockchip: rk3588s-u-boot: Add sdmmc node

ARM: dts: rockchip: rk3588s-u-boot: Add sdmmc node

Booting from SDMMC is one of the fast and easy booting methods
for initial support of any SoC to upstream more features. 

This patch is trying to add the sdmmc node for rk3588 and added
as u-boot specific node in -u-boot.dtsi as upstream Linux is not
supporting yet.

As soon as Linux supports it, a sync of the Linux device tree
would eventually drop this node. 

Clock properties as added according to the rockchip mmc driver
but the actual definition might add scmi clocks into 0 and 1
indexes. This is due to scmi clock are not supporting in upstream
U-Boot. Properly addition of scmi clock would eventually follow
sdmmc clock definition of Linux once they upstreamed.

Signed-off-by: Jagan Teki <jagan@edgeble.ai>