[media] adv7842: Make output format configurable through pad format operations

Replace the dummy video format operations by pad format operations that
configure the output format.

Copied from the adv7604 driver.

Note: while arch/blackfin/mach-bf609/boards/ezkit.c uses adv7842_platform_data
this source has not been updated because it is broken since the very
beginning. It depends on a struct adv7842_output_format that does not
exist.

And besides that gcc has no support for bf609 so nobody can compile it
except by installing a toolchain from ADI.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>