arm64: dts: rockchip: add pinctrls for 16-bit/18-bit rgb interface to rk356x

arm64: dts: rockchip: add pinctrls for 16-bit/18-bit rgb interface to rk356x

The rk3568-pinctrl.dtsi only defines the 24-bit RGB interface. Add separate
nodes for the 16-bit and 18-bit version, respectively. While at it, split
off the clock/sync signals from the data signals.

The exact mapping of the data pins was discussed here:
https://lore.kernel.org/linux-rockchip/f33a0488-528c-99de-3279-3c0346a03fd6@wolfvision.net/T/

Signed-off-by: Michael Riesch <michael.riesch@wolfvision.net>
Link: https://lore.kernel.org/r/20230124054706.3921383-7-michael.riesch@wolfvision.net
Signed-off-by: Heiko Stuebner <heiko@sntech.de>