Convert hi3660 struct comments to kernel-doc notation and fix
other kernel-doc warnings:

drivers/mailbox/hi3660-mailbox.c:47: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
    * Hi3660 mailbox channel information
drivers/mailbox/hi3660-mailbox.c:62: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
    * Hi3660 mailbox controller data
hi3660-mailbox.c:53: warning: contents before sections
hi3660-mailbox.c:67: warning: contents before sections

Fixes: 41c0e939 ("mailbox: Add support for Hi3660 mailbox")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Ruyi Wang <wangruyi@huawei.com>
Cc: Kaihua Zhong <zhongkaihua@huawei.com>
Reviewed-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>