[media] em28xx: extend the support for device buttons

The current code supports only a single snapshot button assigned to
register 0x0c bit 5. But devices may be equipped with multiple buttons
with different functionalities and they can also be assigned to the
various GPI-ports.
Extend the em28xx-input code to handle multiple buttons assigned to different
GPI-ports / register addresses and bits.
Also make easier to extend the code with further button types.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>