fix(versal): resolve misra 8.13 warnings

MISRA Violation: MISRA-C:2012 R.8.13
- The pointer variable points to a non-constant type
but does not modify the object it points to. Consider
adding const qualifier to the points-to type.

Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@xilinx.com>
Change-Id: I74e1b69290e081645bec8bb380128936190b5e24
diff --git a/plat/xilinx/common/ipi_mailbox_service/ipi_mailbox_svc.c b/plat/xilinx/common/ipi_mailbox_service/ipi_mailbox_svc.c
index f531158..a46ac80 100644
--- a/plat/xilinx/common/ipi_mailbox_service/ipi_mailbox_svc.c
+++ b/plat/xilinx/common/ipi_mailbox_service/ipi_mailbox_svc.c
@@ -64,7 +64,7 @@
  * function with rt_svc_handle signature
  */
 uint64_t ipi_smc_handler(uint32_t smc_fid, uint64_t x1, uint64_t x2,
-			 uint64_t x3, uint64_t x4, void *cookie,
+			 uint64_t x3, uint64_t x4, const void *cookie,
 			 void *handle, uint64_t flags)
 {
 	int ret;