media: allegro: rework mbox handling

Add a send/notify abstraction for the mailbox and separate the message
handling in the driver from the code to read and write message to the
mailbox.

This untangles how mails are written into the MCU's SRAM and signaled to
the MCU from the protocol between the driver and the firmware.

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>