[media] adv7604/ad9389b/ths8200: decrease min_pixelclock to 25MHz

The CEA-861 standard allows for the 640x480 format at 25.175 MHz.
Ensure that that's allowed according to the struct v4l2_bt_timings_cap.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>