dt-bindings: mailbox: add google,gs101-mbox

Add bindings for the Samsung Exynos Mailbox Controller.

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>