We want to the higher speed for wifi module working.

Bootup kernel log:
...
mmc_host mmc0: Bus speed (slot 0) = 37125000Hz (slot req 37500000Hz,
actual 37125000HZ div = 0)

or run 'cat /sys/kernel/debug/clk/clk_summary |grep phase -C 1' to check
Otherwise, the mmc0 will run 400khz defalult value to work.
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>