arm64: zynqmp: Changed scratch address used by the alternate memory test

This patch changed CONFIG_SYS_MEMTEST_SCRATCH address to the
accessible DDR address used by alternate memory test.
Before this, 0xfffc0000 address was used, which is the OCM
address and not enabled in MMU table. So, whenever trying
to access 0xfffc0000 address, got Synchronous Abort exception.

After changing CONFIG_SYS_MEMTEST_SCRATCH address, alternate
memory test is working fine.

Signed-off-by: Vipul Kumar <vipulk@xilinx.com>
Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>