boards: qemu_cortex_r5: Reduce memory footprint when running

When running in qemu we utilize fdt-single_arch-zcu102-arm.dtb to
configure qemu.  The current dtb requests just under 2G of memory in the
simulation, but we don't really need this and it impacts running on low
memory VMs.

So remove DDR ddr_bank2 and reduce ddr_bank1_3 from 1G - 256K to
128M - 256K in size.

Signed-off-by: Kumar Gala <[email protected]>

Author: galak