dt-bindings: add compatible strings for big/little rockchip vops

This patch documents the compatible strings for the big and little vop
in rockchip's drm driver.

Cc: Rob Herring <[e-mail]>
Cc: Pawel Moll <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Ian Campbell <[e-mail]>
Cc: Kumar Gala <[e-mail]>

Reviewed-by: Tomasz Figa <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Mark Yao <[e-mail]>
[seanpaul removed superfluous description per tfiga's review]
Signed-off-by: Sean Paul <[e-mail]>

Documentation/devicetree/bindings/display/rockchip/rockchip-vop.txt

···
 - compatible: value should be one of the following
 		"rockchip,rk3036-vop";
 		"rockchip,rk3288-vop";
+		"rockchip,rk3399-vop-big";
+		"rockchip,rk3399-vop-lit";
 
 - interrupts: should contain a list of all VOP IP block interrupts in the
 		 order: VSYNC, LCD_SYSTEM. The interrupt specifier