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: 9ef4546cbd7e ("mailbox: Add support for ST's Mailbox IP")
Signed-off-by: Randy Dunlap <[e-mail]>
Reported-by: kernel test robot <[e-mail]>
Cc: Aditya Srivastava <[e-mail]>
Cc: Lee Jones <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/mailbox-sti.c

···
 #define MBOX_BASE(mdev, inst)   ((mdev)->base + ((inst) * 4))
 
 /**
- * STi Mailbox device data
- *
- * An IP Mailbox is currently composed of 4 instances
- * Each instance is currently composed of 32 channels
- * This means that we have 128 channels per Mailbox
- * A channel an be used for TX or RX
+ * struct sti_mbox_device - STi Mailbox device data
  *
  * @dev:	Device to which it is attached
  * @mbox:	Representation of a communication channel controller
···
  * @name:	Name of the mailbox
  * @enabled:	Local copy of enabled channels
  * @lock:	Mutex protecting enabled status
+ *
+ * An IP Mailbox is currently composed of 4 instances
+ * Each instance is currently composed of 32 channels
+ * This means that we have 128 channels per Mailbox
+ * A channel an be used for TX or RX
  */
 struct sti_mbox_device {
 	struct device		*dev;
···
 };
 
 /**
- * STi Mailbox platform specific configuration
+ * struct sti_mbox_pdata - STi Mailbox platform specific configuration
  *
  * @num_inst:	Maximum number of instances in one HW Mailbox
  * @num_chan:	Maximum number of channel per instance
···
 };
 
 /**
- * STi Mailbox allocated channel information
+ * struct sti_channel - STi Mailbox allocated channel information
  *
  * @mdev:	Pointer to parent Mailbox device
  * @instance:	Instance number channel resides in