dt-bindings: mailbox: Update doc with NSP PDC/mailbox support

Update the DT bindings documentation to reflect the new NSP version
of PDC driver compatibility string.

Signed-off-by: Steve Lin <steven.lin1@broadcom.com>
Acked-by: Jon Mason <jon.mason@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>