V4L/DVB (11742): TI THS7303 video amplifier driver code

This patch adds driver for TI THS7303 video amplifier. This driver is
implemented as a v4l2 sub device. Tested on TI DM646x EVM.

This version has updates based on review comments by Mauro Chehab.

Signed-off-by: Chaithrika U S <chaithrika@ti.com>
Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>