drm/rockchip: vop: Add output selection registers for RK312x

In contrast to RK3036, RK312x SoCs have multiple output channels such as
RGB (i.e. LVDS TTL), LVDS, DSI and HDMI.

In order to support that, this splits output from RK3036 and defines an
separate one for RK3126 with the registers required to enable the
appropriate output and setup the correct polarity.

Signed-off-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20231222174220.55249-3-knaerzche@gmail.com