Enable BCM2835 mailbox support

This patch set is an implementation of BCM2835 mailbox support built on top of the framework implemented by by Suman Ana and Jassi Brar. It is also meant to be a validation of the framework against another platform.

In order to be an effective test, the patch set also includes a mailbox client - a driver for the built-in CPU thermal sensor, which gets readings via a mailbox request and reply.

Both the mailbox driver [PATCH 2/3] and the thermal driver [PATCH 3/3] should be complete. However, [PATCH 1/3] is not a formal part of this submission. It is only included to simplify testing of patches 2 and 3. This is why this patch set is only RFC; it cannot really proceed further until without the generic mailbox support.

Cheers,
Craig.