drm/tilcdc: add a da850-specific compatible string

Due to some potential tweaks for the da850 LCDC (for example: the
required memory bandwith settings) we need a separate compatible
for the IP present on the da850 boards.

Suggested-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jyri Sarha <jsarha@ti.com>