platform: openpiton: Read the device configurations from device tree

OpenPiton is designed to run on different FPGAs with different
configurations. Use the fdt parser to retrieve the required values
from device tree instead of using fixed values.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Tested-by: Jonathan Balkind <jbalkind@cs.princeton.edu>
Reviewed-by: Anup Patel <anup.patel@wdc.com>