arch: arm: cortex_m: do not read MMFAR if MMARVALID is not set

When the MMARVALID bit is not set, do not read the MMFAR
register to get the fault address in a MemManage fault.
This change prevents the fault handler to erroneously
assume MMFAR contains a valid address.

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