soc: infineon: edge: pse84: Clean up the init process on cm33/ns

This commit introduces 2 changes:
- cleans up the mpu initialziation, the old function is not
required because the initialziation is done properly via
mpu_regions.c for all cores
- defaults to using SystemCoreClockSetup instead of
SystemCoreClockUpdate because the former takes the
input from DT whereas the latter would read the info
from the HF clocks which in case of TF-M are secured
and not directly accessible from the cm33/ns (if SRF is
not set up)

Signed-off-by: Laura Carlesso <laura.carlesso@infineon.com>