When sd/mmc work at DDR mode, like HS400/HS400ES/DDR52/DDR50 mode,
the output clock rate is half of the internal clock rate.

This patch set the DDR_EN bit first for DDR mode, hardware divide
the usdhc clock automatically, then follow the original sdr clock
setting method.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Signed-off-by: Ye Li <ye.li@nxp.com>