media: allegro: move mail definitions to separate file

Move the mail definitions from the driver core to a dedicated file.

The mails that are exchanged between driver and firmware are not stable
across firmware versions. This is in preparation to make the driver able
to handle multiple firmware version by having dedicated code for
handling mails.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>