ARM64: zynqmp: Read RAM information from DT

Read information about memory from DT. This patch simplify life with
synchronization between DT and board files.

dram_init() only needs maximum RAM size below 4GB that's why please sort
banks in memory node.
dram_init_banksize() copies memory setup to bi_dram[].
This will avoid reading information from DT twice.

Memory test start/end were changed to DDR location to let memtest still
compiled.

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