phy: Add driver for rockchip Display Port PHY

Add phy driver for the Rockchip DisplayPort PHY module. This
is required to get DisplayPort working in Rockchip SoCs.

Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>