mailbox: Add Tegra HSP driver

This driver exposes a mailbox interface for interprocessor communication
using the Hardware Synchronization Primitives (HSP) module's doorbell
mechanism. There are multiple HSP instances and they provide additional
features such as shared mailboxes, shared and arbitrated semaphores.

A driver for a remote processor can use the mailbox client provided by
the HSP driver and build an IPC protocol on top of this synchronization
mechanism.

Based on work by Joseph Lo <josephl@nvidia.com>.

Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>