ipm: add demo code for inter-processor mailboxes

Migrated from internal repository.
Run "make flash" for each on the same Arduino 101 device.

Origin: Original code, Zephyr "hello world" used as a starting point
Change-Id: I3fdeed6b7e85ad703983c2674e265c85a365ce5b
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>