mailbox: correct kerneldoc

Correct kerneldoc warnings like:

  drivers/mailbox/arm_mhu_db.c:47:
    warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
  drivers/mailbox/qcom-ipcc.c:58:
    warning: Function parameter or member 'num_chans' not described in 'qcom_ipcc'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>