dt-bindings: mailbox: Add interrupt-names to SPRD mailbox

We add an optional supp-outbox interrupt support to SPRD mailbox driver
with newly added sc9863a support and change to configure interrupts with
names in device tree files.

Signed-off-by: Orson Zhai <orson.zhai@unisoc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>