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.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>