drm/tilcdc: add encoder slave (v2)

Add output panel driver for i2c encoder slaves.

v1: original
v2: add DT bindings docs, and minor updates for review comments

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Koen Kooi <koen@dominion.thruhere.net>

drivers/gpu/drm/tilcdc/Makefile

@@ -3,6 +3,7 @@ ccflags-y := -Iinclude/drm -Werror
 tilcdc-y := \
 	tilcdc_crtc.o \
 	tilcdc_tfp410.o \
+	tilcdc_slave.o \
 	tilcdc_drv.o
 
 obj-$(CONFIG_DRM_TILCDC)	+= tilcdc.o