devicetree: Add mailbox bindings

Add general mboxes, mbox-names to base.yaml to be utilized by any
clients that use mailboxes.

Additionally add mailbox-controller.yaml for common properties shared
by all mailbox controller devices.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
Signed-off-by: Carlo Caione <ccaione@baylibre.com>
2 files changed