arm64: dts: rockchip: Add 12V DCIN regulator to rk3399-ficus

12V DCIN regulator is root source supply for the rest of regulators
in Rock960 power diagram.

Add support for it and attach same to supply vcc5v0_sys.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Akash Gajjar <akash@openedev.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>