rockpro64: Enable HDMI output on rockpro64 board

Reference to commit that adds HDMI to other rk3399 boards:
commit 9778edae ("rockchip: Enable HDMI output on rk3399 board w/ HDMI")

Signed-off-by: Andrius Štikonas <andrius@stikonas.eu>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>