mailbox: zynqmp: Fix typo in IPI documentation

Xilinx IPI message buffers allows 32-byte data transfer.
Fix documentation that says 12 bytes

Fixes: 4981b82ba2ff ("mailbox: ZynqMP IPI mailbox controller")
Signed-off-by: Tanmay Shah <[e-mail]>
Acked-by: Michal Simek <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mathieu Poirier <[e-mail]>

include/linux/mailbox/zynqmp-ipi-message.h

···
  * @data: message payload
  *
  * This is the structure for data used in mbox_send_message
- * the maximum length of data buffer is fixed to 12 bytes.
+ * the maximum length of data buffer is fixed to 32 bytes.
  * Client is supposed to be aware of this.
  */
 struct zynqmp_ipi_message {