We need to introduce snd_sof_mailbox_{read/write} in order to provide
a generic way for mailbox access. These routines are optional, each
platform can implement their own specific routines.

So far, all platforms use mmapped I/O thus they can use custom made
routines sof_mailbox_read / sof_mailbox_write that use MMIO.
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Bud Liviu-Alexandru <budliviu@gmail.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://lore.kernel.org/r/20211004152147.1268978-2-daniel.baluta@oss.nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>