rockchip: rk322x: update max-frequency for mmc node

mmc using 150000000 as max-frequency like what rk3288 sets.
This can speed up the mmc read/write, the actual mmc clock is:
Before this patch: 37.125M
After this patch: 49.5M

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>