media: uvcvideo: Add Privacy control based on EXT_GPIO

Add a new control and mapping for Privacy controls connected to
UVC_GUID_EXT_GPIO_CONTROLLERs.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>