mailbox: Enable BCM2835 mailbox support

This mailbox driver provides a single mailbox channel to write 32-bit
values to the VPU and get a 32-bit response.  The Raspberry Pi
firmware uses this mailbox channel to implement firmware calls, while
Roku 2 (despite being derived from the same firmware tree) doesn't.

The driver was originally submitted by Lubomir, based on the
out-of-tree 2708 mailbox driver.  Eric Anholt fixed it up for
upstreaming, with the major functional change being that it now has no
notion of multiple channels (since that is a firmware-dependent
concept) and instead the raspberrypi-firmware driver will do that
bit-twiddling in its own messages.
[Jassi: made the 'mbox_chan_ops' struct as const and removed a redundant
variable]

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Craig McGeachie <slapdau@yahoo.com.au>
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>