arm64: dts: rockchip: set sdmmc0 speed to sd-uhs-sdr50 on rock-3a

As other rk336x based devices, the Rock 3 Model A has issues with high
speed SD cards, so lower the speed to 50 instead of 104 in the same
manor has the Quartz64 Model B has.

Fixes: 22a442e6

 ("arm64: dts: rockchip: add basic dts for the radxa rock3 model a")
Signed-off-by: Dan Johansen <strit@manjaro.org>
Link: https://lore.kernel.org/r/20230128112432.132302-1-strit@manjaro.org
Signed-off-by: Heiko Stuebner <heiko@sntech.de>