arch: arm: correct fault address logging

In ARMv7-M (and ARMv8-M) architecture it is
implementation defined whether separate MMFAR and BFAR are
implemented. This commit ensures that we always get the true
faulting address displayed in case of MemManage- or BusFault.

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