arm64: zynqmp: Update scriptaddr value to 512MB

This patch updates scriptaddr value to 512MB as having it at 32MB has high
chance of script corruption incase of bigger kernel.

Be aware that 512MB is used for SPL malloc area that's why images should be
loaded after SPL (the best after u-boot relocation).

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>