arm64: dts: rockchip: add eMMC's power domain support for rk3399

Control power domain for eMMC via genpd to reduce power consumption.

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Tested-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>