boards: arm: qemu_cortex_m0: use LFXTAL

Update configuration of qemu_cortex_m0 to use LF XTAL as low
frequency clock source. Previously used RC requires more
modules to be fetched into the build, including TEMP sensor
used for clock calibration.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>