boards: arm: mps2_an521: Enable QEMU icount mode

This commit enables the QEMU icount mode for `mps2_an521`, in order to
decouple the host clock from the emulated guest clock.

This prevents guest timing instability from causing test failures when
the host CPU load is very high.

The icount `shift` value of 7 was empirically chosen to allow the tests
to complete in both realistic and reasonable amount of time.

The following are quick notes on the parameters used:

* -icount shift=7: Execute one instruction every 128ns of virtual time
* -icount align=off: Do not synchronise the host and guest clocks
* -icount sleep=off: Advance virtual time without sleeping/waiting
* -rtc clock=vm: Isolate the guest RTC time from the host

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>