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/docs/plat/xilinx-zynqmp.rst b/docs/plat/xilinx-zynqmp.rst
index 81f4fbe..4fe0d2f 100644
--- a/docs/plat/xilinx-zynqmp.rst
+++ b/docs/plat/xilinx-zynqmp.rst
@@ -89,6 +89,16 @@
 make CROSS_COMPILE=aarch64-none-elf- PLAT=zynqmp RESET_TO_BL31=1 DEBUG=1 \
 	ZYNQMP_ATF_MEM_BASE=0x40000 ZYNQMP_ATF_MEM_SIZE=<size>
 
+Configurable Stack Size
+-----------------------
+
+The stack size in TF-A for ZynqMP platform is configurable.
+The custom package can define the desired stack size as per the requirement in
+the make file as follows,
+
+PLATFORM_STACK_SIZE := <value>
+$(eval $(call add_define,PLATFORM_STACK_SIZE))
+
 FSBL->TF-A Parameter Passing
 ----------------------------