arm64: dts: rockchip: slow down emmc freq for rock 5 itx

arm64: dts: rockchip: slow down emmc freq for rock 5 itx

The current max-frequency 200000000 of emmc is not stable. When doing
heavy write there will be I/O Error. After setting max-frequency to
150000000 the emmc is stable under write.

Also remove property mmc-hs200-1_8v because we are already running at
HS400 mode.

Tested with fio command:
fio -filename=./test_randread -direct=1 -iodepth 1 -thread \
-rw=randwrite -ioengine=psync -bs=16k -size=1G -numjobs=10 \
-runtime=600 -group_reporting -name=mytest

Signed-off-by: Jianfeng Liu <liujianfeng1994@gmail.com>
Link: https://lore.kernel.org/r/20250228143341.70244-1-liujianfeng1994@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>