drm/tilcdc: Remove "default" from blue-and-red-wiring property binding

Remove "default" keyword from blue-and-red-wiring devicetree property
binding document. The code does not support and there is no intention
to support it.

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jyri Sarha <jsarha@ti.com>