Add HS400 support for iMX7ULP B0.

According to IC suggest, need to clear the STROBE_DLL_CTRL_RESET
before any setting of STROBE_DLL_CTRL register.

USDHC has register bits(bit[27~20] of register STROBE_DLL_CTRL)
for slave sel value. If this register bits value is 0,  it needs
256 ref_clk cycles to update slave sel value. IC suggest to set
bit[27~20] to 0x4, it only need 4 ref_clk cycle to update slave
sel value. This will short the lock time of slave.

i.MX7ULP B0 will need more time to lock the REF and SLV, so change
to add 5us delay.
Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>