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>