boards: arm: qemu_cortex_a9: change board's timing parameters

- Fix the system clock frequency: should be 111.1 MHz instead
of 100 MHz.
- Set ticks per second to 1000 for higher system clock precision.
- Set QEMU icount shift value to 3 so that one instruction gets
executed every 2^3 = 8 ns.

Signed-off-by: Immo Birnbaum <Immo.Birnbaum@Weidmueller.com>