Documentation: bindings: add dt documentation for Rockchip eMMC PHY

This patch adds a binding that describes the Rockchip eMMC PHYs
found on Rockchip SoCs eMMC interface.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>