arm64: dts: imx8mn: Move usdhc clocks assignment to board DT

usdhc's clock rate is different according to different devices
connected, so clock rate assignment should be placed in board
DT according to different devices connected on each usdhc port.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>