dt: mailbox: Remove 'mbox-names property is discouraged' message from binding

- 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.

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

Example:

	pwr_cntrl: power {

		...

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

		mboxes = <&mailbox 0

			&mailbox 1>;

		mboxes = <&mailbox 0 &mailbox 1>;

	};