mailbox: Add support for Armada 37xx rWTM mailbox

This adds support for the mailbox via which the kernel can communicate
with the firmware running on the secure processor of the Armada 37xx
SOC.

The rWTM secure processor has access to internal eFuses and
cryptographic circuits, such as the Entropy Bit Generator to generate
true random numbers.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>