xtos: move sof/lib/mailbox.h to application interface

The SOF mailbox.h provides an interface to host-DSP mailboxes, which is
used widely in SOF IPC and debug code.

This interface is OS agnostistic and only relies on platform/lib/mailbox.h
to define the mailbox locations, and rtos/cache.h to define portable cache
primitives to invalidate/writeback mailbox data before/after use.

The amount of RTOS variation does not seem to warrant branching the
whole mailbox.h to RTOS layer. Move mailbox.h back to application
interface, so the single implementation can be shared.

Link: #9015
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>