logging: Intel ADSP mtrace logging backend

Implements a logging backend implementing the mtrace protocol
to relay log messages over SRAM. This protocol is implemented
by the SOF Linux driver ("mtrace" debugfs interface).

This implementation follows example of the ADSP HDA logger implemented
in commit 6913da9d ("logging: cAVS HDA based logger").

Cc: Tom Burdick <thomas.burdick@intel.com>
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>