[media] TW686x frame grabber driver

A driver for Intersil/Techwell TW686x-based PCIe frame grabbers.

[hans.verkuil@cisco.com: renamed staging tw686x to tw686x-kh to prevent naming conflicts]
[hans.verkuil@cisco.com: don't build tw686x-kh if tw686x is already selected to prevent conflicts]
[mchehab@osg.samsung.com: use "unsigned int" instead of just "unsigned"  and add some whitespaces to make checkpatch happier]
Signed-off-by: Krzysztof Halasa <khalasa@piap.pl>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Hans Verkuil <hans.verkuil@cisco.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>