boards: qemu: cortex_r5: expand usable RAM area below 4G boundary

Expand the RAM area beginning behind the BTCM to allow the use
of a 64MB SRAM area starting at 0x4000000, which is the lowest
possible 64MB area with a non-zero start address that doesn't
overlap with the ATCM and BTCM memory areas.

Signed-off-by: Immo Birnbaum <mail@birnbaum.immo>