mailbox: sti: quieten kernel-doc warnings

Use kernel-doc struct notation for the mailbox structs to prevent
these kernel-doc warnings:

   drivers/mailbox/mailbox-sti.c:39: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
    * STi Mailbox device data
   drivers/mailbox/mailbox-sti.c:63: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
    * STi Mailbox platform specific configuration
   drivers/mailbox/mailbox-sti.c:74: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
    * STi Mailbox allocated channel information

Also move the field descriptions ahead of the function description as
is expected in kernel-doc. This prevents another kernel-doc warning.

Fixes: 9ef4546c ("mailbox: Add support for ST's Mailbox IP")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Aditya Srivastava <yashsri421@gmail.com>
Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>