mmc: rockchip_sdhci: enable strobe line for HS400

The default configuration of rk3399 EMMC PHY does not enable the
strobe line, and EMMC controller will got data transmission error
at HS400 mode.

Signed-off-by: Yifeng Zhao <yifeng.zhao@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>