cpu: Factor the page size out of the decode cache.

There isn't anything special about using the page size, and it creates
an artificial dependence on the ISA. Instead of being based on pages,
the cache is now based on "chunks" who's size is a template parameter.
It defaults to 4K which is a common page size, but it can be tuned
arbitrarily if necessary.

Some unnecessary includes have been trimmed out as well.

Change-Id: I9fe59a5668d702433a800884fbfbdce20e0ade97
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33204
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>