arc: add ram to qemu board yaml so arc tests run

With a missing RAM value in the ARC QEMU board YAML,
Twister assigns it 128. The min_ram filter for the
testcase is higher. The RAM in the ARC QEMU board DTS
is 4 MB. Update ARC QEMU board YAML to include 4 MB
of RAM so tests run on ARC QEMU.

Signed-off-by: Lauren Murphy <lauren.murphy@intel.com>