Add output panel driver for i2c encoder slaves.

v1: original
v2: add DT bindings docs, and minor updates for review comments
Signed-off-by: NRob Clark <robdclark@gmail.com>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: NKoen Kooi <koen@dominion.thruhere.net>