Move PMU functions from pmu_ethosu.h to ethosu_pmu.c

This change is done as a prerequisite to be able to set
base address for NPU through ethosu_init(). So instead of using
a mamory mapped struct we use the same functions to read/write
PMU registers as the rest of the ethos-u driver.

Change-Id: Ie1f6e6af4627cbe9375e2b2d079ba8017b6294d3