dt-bindings: display: rockchip: make reg-names mandatory for VOP2

The VOP2 driver relies on reg-names properties, but these are not
documented. Add the missing documentation and make reg-names mandatory.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220511082109.1110043-2-s.hauer@pengutronix.de