mailbox: Use of_property_match_string() instead of open-coding

Use of_property_match_string() instead of open-coding the search. With
this, of_get_property() can be removed as there is no need to check for
"mbox-names" presence first.

This is part of a larger effort to remove callers of of_get_property()
and similar functions. of_get_property() leaks the DT property data
pointer which is a problem for dynamically allocated nodes which may
be freed.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>