drm/rockchip: Add crtc_endpoint_id to rockchip_encoder

The VOP2 has an interface mux which decides to which encoder(s) a CRTC
is routed to. The encoders and CRTCs are connected via of_graphs in the
device tree. When given an encoder the VOP2 driver needs to know to
which internal register setting this encoder matches. For this the VOP2
binding offers different endpoints, one for each possible encoder. The
endpoint ids of these endpoints are used as a key from an encoders
device tree description to the internal register setting.

This patch adds the key aka endpoint id to struct rockchip_encoder plus
a function to read the endpoint id starting from the encoders device
node.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220422072841.2206452-4-s.hauer@pengutronix.de