boards/qemu_cortex_a53: enable qemu icount mode

This is a single-CPU board that would normally be expected to have
icount enabled, but doesn't.  So it sees a ton of false positive test
failures due to timing skew, which (for still-poorly-understood
reasons) have recently become much worse in CI.

It looks like this got held back by a qemu bug where the emulator
would ignore SIGTERM, but that can be worked around in twister.

Turn it on.  The SHIFT value of 4 matches the 62.5 MHz simulated
clock exactly, which is pleasing.

Signed-off-by: Andy Ross <andyross@google.com>