boards: arm: qemu_cortex_m0: implement custom system clock driver

For the qemu_cortex_m0 we implement a custom system clock
driver based on the nRF51 TIMER peripheral. The system
clock is configured to run at 1 MHz frequency.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>