boards: arm: qemu_cortex_m0: configure board with own timer driver

This commit configures the qemu_cortex_m0 board to
build with it's custom timer driver, instead of the
default nrf_rtc_timer driver for nRF51x SoCs. It,
additionally, configures a default system clock
frequency to 1MHz, as well as 10 Hz tick frequency.

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