arm64: zynqmp: Set initrd_high to as high as possible

This patch is setting up the initrd_high to as high as possible by leaving
max stack size for u-boot so that bigger rootfs can also be loaded by
u-boot for booting kernel.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>