* 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.

Example:
	pwr_cntrl: power {
		...
		mbox-names = "pwr-ctrl", "rpc";
		mboxes = <&mailbox 0 &mailbox 1>;
	};
