rockchip: rk3588-rock-5b: Include eMMC node in SPL dtb

Add sdhci node to SPL and u-boot,spl-boot-order. Also add more supported
mmc modes and pinctrl.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>