media: i2c: ds90ub953: Add TPG support

Add TPG support using an internal pad. For now, only RGB888 with red,
green or blue patterns are supported. Setting frame interval is
supported.

The TPG can be enabled, when streaming is off, by setting a routing with
a single stream from the internal TPG pad to the source pad. The TPG
pattern can be changed even when the streaming is eanbled.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>