cpu: Fixed useful counter handling in LTAGE

cpu: Fixed useful counter handling in LTAGE

Increased to 2 bits of useful counter per TAGE entry as described in the
LTAGE paper (and made the size configurable)

Changed how the useful counters are incremented/decremented as described
in the LTAGE paper

Change-Id: I8c692cc7c180d29897cb77781681ff498a1d16c8
Signed-off-by: Pau Cabre <pau.cabre@metempsy.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14215
Reviewed-by: Ilias Vougioukas <ilias.vougioukas@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>