[media] em28xx: add basic support for OmniVision OV2640 sensors

This sensor is used by the "SpeedLink Vicious And Devine Laplace webcam" and
others. It supports resolutions up to 1600x1200 (at 7-8 fps), but for
resolutions higher than 640x480, further driver changes will be necessary,
such as sensor output resolution switching (including further configuration
changes), bridge xclk adjustment and disabling of 16 bit (12 bit) output formats
at high resolutions. Image quality should also needs to be improved.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>