configs: Do not require default options for caches

configs: Do not require default options for caches

This change is useful when using custom simulation scripts that do not
rely on configs/common/Options.py.
Without this change, the custom script always needed to provide some
value for cache sizes and HW prefetchers configuration; with this change
it is possible to provide no value and use what is defined in the core
configuration as default.

Change-Id: Id0e807c3fa224180d682f366c7307941bab8ce59
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36776
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>