dt-bindings: mailbox: Add thead,th1520-mailbox bindings

Add bindings for the mailbox controller. This work is based on the vendor
kernel. [1]

Link: https://github.com/revyos/thead-kernel.git

 [1]

Signed-off-by: Michal Wilczynski <m.wilczynski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>