boards: qemu: cortex_r5: declare 64MB sram0 area at board level

Declare the sram0 area for this board to start at 0x4000000
and to be 64MB in size. The base address is the lowest possible
multiple of 64MB which doesn't have its base address at 0,
therefore preventing the mixed use of regular RAM and the
ATCM/BTCM and the possible issues of having a 'black hole'
between the ATCM and BTCM areas or the BTCM being completely
disabled.

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