ARM: zynq: Setup stack size via Kconfig

Stack size has been introduced by commit a69814c8 ("arm64: zynqmp:
Set initrd_high to as high as possible") and commit 085201c2 ("arm64:
versal: Set initrd_high to as high as possible")
to support setting up initrd_high as high as possible.
The same change should happen for Zynq because the code is moved to xilinx
common location.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>