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

A new API call has been introduced which allows channels to be
requested by name.  This new call uses the 'mbox-names' property,
so users need no further discouragement from supplying it.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>