config: Make some MemConfig options optional

config: Make some MemConfig options optional

MemConfig currently assumes that all callers include the its full set
of options in the command line parser. This is unnecessary and
sometimes confusing. Make most of the options optional to avoid having
to add all of them to example scripts.

Change-Id: I2d73be2454427b00db16716edcfd96a47133c888
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Gabor Dozsa <gabor.dozsa@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3940
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>