Switch Tock's mailbox driver to use the RTL register map.

The previous implementation was a placeholder, now that we have a
design doc and register map for the hardware mailbox we can make the
two match up.

Additionally, clean up the driver's 'wait for mailbox event' handling.

Change-Id: I5aa43a112cef9e810dc6028a5dba327c4c4924a1
2 files changed