dt: mailbox: add generic bindings

* Generic Mailbox Controller and client driver bindings

Generic binding to provide a way for Mailbox controller drivers to

assign appropriate mailbox channel to client drivers.

* Mailbox Controller

Required property:

- #mbox-cells: Must be at least 1. Number of cells in a mailbox

		specifier.

Example:

	mailbox: mailbox {

		...

		#mbox-cells = <1>;

	};

* Mailbox Client

Required property:

- mboxes: List of phandle and mailbox channel specifiers.

Optional property:

- mbox-names: List of identifier strings for each mailbox channel

		required by the client. The use of this property

		is discouraged in favor of using index in list of

		'mboxes' while requesting a mailbox. Instead the

		platforms may define channel indices, in DT headers,

		to something legible.

Example:

	pwr_cntrl: power {

		...

		mbox-names = "pwr-ctrl", "rpc";

		mboxes = <&mailbox 0

			&mailbox 1>;

	};