feat(zynqmp): make stack size configurable

If PLATFORM_STACK_SIZE not already defined, use the default value of
PLATFORM_STACK_SIZE.
This makes the stack size value configurable for different interface
like custom packages.

Signed-off-by: Amit Nagal <amit.nagal@amd.com>
Signed-off-by: Akshay Belsare <akshay.belsare@amd.com>
Change-Id: I87e9fcbfb4c4092378b1ac0ff8fb6d084495d320
diff --git a/plat/xilinx/zynqmp/include/platform_def.h b/plat/xilinx/zynqmp/include/platform_def.h
index aebce30..fb1130f 100644
--- a/plat/xilinx/zynqmp/include/platform_def.h
+++ b/plat/xilinx/zynqmp/include/platform_def.h
@@ -21,7 +21,9 @@
  ******************************************************************************/
 
 /* Size of cacheable stacks */
+#ifndef PLATFORM_STACK_SIZE
 #define PLATFORM_STACK_SIZE 0x440
+#endif
 
 #define PLATFORM_CORE_COUNT		U(4)
 #define PLAT_NUM_POWER_DOMAINS		U(5)