mailbox: allow subnode for mbox regs

The following patch allows the mailbox node in DT to host subnodes with
mailbox definitions. If the client phandle to the mailbox is not the
mailbox driver node, just checks parents as well.

Signed-off-by: Ibai Erkiaga <ibai.erkiaga-elorza@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>