phy: add a driver for the Rockchip SoC internal eMMC PHY

This patch to add a generic PHY driver for ROCKCHIP eMMC PHY.
Access the PHY via registers provided by GRF (general register
files) module.

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