drm/tilcdc: Add DRM_TILCDC_SLAVE_COMPAT for ti,tilcdc,slave binding support

Adds a CONFIG_DRM_TILCDC_SLAVE_COMPAT module for "ti,tilcdc,slave"
node conversion. The implementation is in tilcdc_slave_compat.c and it
uses tilcdc_slave_compat.dts as a basis for creating a DTS
overlay. The DTS overlay adds an external tda998x encoder to tilcdc
that corresponds to the old tda998x based slave encoder.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>