soc: nordic: Use default SYS_CLOCK_TICKS_PER_SEC for PPR core

PPR was using 1 kHz system clock frequency instead of default 31250 Hz
used on other cores with GRTC. Low frequency impacts system clock
accuracy. There is no reason to use different frequency for PPR.

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