drm/armada: add plane colorspace properties

Use the DRM standard plane properties for specifying the YUV
colour encoding parameter.  Our colour range is fixed at limited
range.

Since we are transitioning to atomic modeset, we need to explicitly
add handling of these properties to our atomic_set_property() method,
but once the transition is complete, these will be removed.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>