versal: Support DDRMC0 region 0 and 1

[chrisj at rtems.org]

The Versal's DDRMC0 supports two separate regions. Region 0 is
from 0 up to 2G where the Versal's hard IP regions start. DDR
memory above the 2G mark is moved to region 1 and its base
address is in the A64 address space.

The patch will place all memory up to 2G in region 0 and if
more is present it is located in region 1. An MMU entry for the
region 1 memory is provided and the memory is added to the heap.

Chris