arm64: dts: rockchip: Add missing sdmmc2 SDR rates to rock-3a

Add missing UHS-I SDR rates to sdmmc2. Add explicit alias as mmc2 while at it.
It would be good to have matching timings enabled in case slower SDIO devices
are encountered.

Signed-off-by: Tamás Szűcs <tszucs@protonmail.ch>
Link: https://lore.kernel.org/r/20231011191448.58936-1-tszucs@protonmail.ch


Signed-off-by: Heiko Stuebner <heiko@sntech.de>