arm: Replicate the PageBytes constant in the ArmSystem class.

arm: Replicate the PageBytes constant in the ArmSystem class.

When isa_traits.hh hopefully goes away in the not too distant future,
this constant will need somewhere to live so ARM components can find it.
There are valid arguments that this should not be a constant in the
first place, but that's outside the scope of this change.

Change-Id: Ic5bd046dc1cc196b3cf6b6c36878fdbf5eb4c0bf
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34170
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>