rockchip: board: Add minimal generic RK3588S/RK3588 board

Add a minimal generic RK3588S/RK3588 board that only have eMMC and SDMMC
enabled. This defconfig can be used to boot from eMMC or SD-card on most
RK3588S/RK3588 boards that follow reference board design.

Also fix the alphabetical order of RK3588 boards listed in Makefile and
documentation.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>