mailbox: zynqmp: Fix typo in IPI documentation

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

Fixes: 4981b82b ("mailbox: ZynqMP IPI mailbox controller")
Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Acked-by: Michal Simek <michal.simek@amd.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230311012407.1292118-4-tanmay.shah@amd.comSigned-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

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

@@ -9,7 +9,7 @@
  * @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 {