qemu_cortex_a53: Unset QEMU icount to enable MTTCG

Quoting from the QEMU manual "MTTCG is not compatible with -icount and
enabling icount will force a single threaded run.". Given that for
Cortex-A53 we haven't seen any particular problem when disabling icount
try to disable it.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>