media: allegro: add explicit mail encoding and decoding

The message format in the mailboxes differ between firmware versions.
Therefore, it is necessary to decouple the mailbox format of the driver
from the message format of the firmware. This allows to keep a
consistent message format in the driver while still supporting various
firmware versions.

Add an intermediate step to encode and decode message before writing the
mails to the mailboxes.

On the other hand, this allows to handle optional fields in the
messages, which is required for advanced features of the encoder and was
not possible until now.

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>