MLK-10444 ARM: imx7: set usdhc3 clock rate as 400MHz

For i.MX7D, only the usdhc3 support HS400 mode, this is because
HS400 mode need addtional pad: strobe. This pad is used for HS400
read clock reference. And only usdhc3 has this strobe pad on i.MX7D.

HS400 is in DDR mode, the clock will default divide 2 if the usdhc in
DDR mode, so that to make HS400 work at 200MHz, we need to set the
usdhc3 root clock rate at 400MHz at least.

Signed-off-by: Haibo Chen <haibo.chen@freescale.com>