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 <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>

Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
[seanpaul removed superfluous description per tfiga's review]
Signed-off-by: Sean Paul <seanpaul@chromium.org>