dt-bindings: mailbox: add Axiado AX3005 mailbox

Add a devicetree binding for the Axiado AX3005 mailbox controller.
The controller provides inter-processor communication channels
between the host CPU and the coprocessor, with separate TX and RX
register regions.

Add the corresponding MAINTAINERS entry covering the binding.

Signed-off-by: Swark Yang <syang@axiado.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>