arm64: dts: rockchip: enable gamma control on RK3399

Define the memory region on RK3399 VOPs containing the gamma LUT at
base+0x2000.

Signed-off-by: Hugh Cole-Baker <sigmaris@gmail.com>
Tested-by: Linus Heckemann <git@sphalerite.org>
Link: https://lore.kernel.org/r/20211019215843.42718-4-sigmaris@gmail.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>