boards: arm: fvp_base_revc_2xaem: Increase DRAM to 128 MB

Increase FVP Base RevC 2xAEM DRAM from 2 MB to 128 MB to match the
platform's actual capabilities and align with QEMU's configuration.

The FVP Base RevC supports ~4 GB of RAM as documented in the Linux
kernel device tree (arch/arm64/boot/dts/arm/fvp-base-revc.dts), which
defines two memory regions totaling approximately 4 GB.

The previous 2 MB limitation caused issues with demand paging tests
where the kernel VM size (8 MB) exceeded available RAM, triggering
early page faults before the page frame allocator was initialized.
With 128 MB, demand paging workloads have sufficient memory to operate
without premature paging.

This change brings FVP's memory configuration in line with QEMU
Cortex-A53 (128 MB).

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>