drm/bridge: Add sii902x driver

Add basic support for the sii902x RGB -> HDMI bridge.
This driver does not support audio output yet.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Archit Taneja <architt@codeaurora.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
---
Changes in v8:
- remove useless headers inclusion
- fix macro names (s/SIL/SII)
- drop unneeded hotplug_work field from struct sii902x
- drop drm_connector_unregister() call in the ->destroy() method
- add a timeout when polling a register value

Changes in v6:
- use HDMI_INFOFRAME_SIZE(AVI)
- fix reset_gpio initialization
- reduce the reset time based on Ming feedback

Changes in v5:
- drop the best_encoder() implementation

Changes in v4:
- make reset GPIO optional
- only support attaching to DRM devices supporting atomic updates

Changes in v3:
- fix get_modes() implementation to avoid turning the screen in power
  save mode
- rename the driver (sil902x -> sii902x)

Changes in v2:
- fix errors reported by the kbuild robot

fixup! drm: bridge: Add sii902x driver