dts: rockchip: rk3399: enable emmc phy for spl

adapting commit ac804143 ("mmc: rockchip_sdhci: add phy and clock
config for rk3399") to fix the issue "Not found emmc phy device".

Signed-off-by: Yifeng Zhao <yifeng.zhao@rock-chips.com>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com> - on a Rock960
Reviewed-by: Patrick Wildt <patrick@blueri.se>
Tested-by: Patrick Wildt <patrick@blueri.se> - on a Pinebook Pro
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>