drivers: clock control: rename nrf5_power_clock source and header

This commit renames the nrf5_clock_control.h and
nrf5_clock_control.c files to nrf_clock_control.h and
nrf_clock_control.c, respectively, as they are used
in nRF9160 builds, as well.

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