drm: rockchip: dp: add rockchip platform dp driver

Rockchip have three clocks for dp controller, we leave pclk_edp
to analogix_dp driver control, and keep the sclk_edp_24m and
sclk_edp in platform driver.

Acked-by: Mark Yao <mark.yao@rock-chips.com>
Tested-by: Caesar Wang <wxt@rock-chips.com>
Tested-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>