[media] em28xx: extend GPIO register definitions for the em25xx, em276x/7x/8x, em2874/174/84

The em25xx/em276x/7x/8x provides 4 GPIO register sets,
each of them consisting of separate read and a write registers.
The same registers are also used by the em2874/174/84.

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