boards: qemu_cortex_r5: Enable QEMU icount emulation

This commit enables the QEMU icount emulation mode for improved timing
stability.

In normal emulation mode (without icount), the emulation timing of the
TTC system timer is particularly unstable and this results in a high CI
failure rate.

For more details, refer to the issues #14173 and #22904.

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