mem-cache: Remove isTouched field from the CacheBlk

mem-cache: Remove isTouched field from the CacheBlk

At the moment isTouched is used in the warm-up detection mechanism but
it keeps track of the same information as isValid(). This change
removes it and substitutes its use by isValid().

Change-Id: I611ddf2fa4562ae3b3b2ed2fb74d26abd2e5ec62
Reviewed-on: https://gem5-review.googlesource.com/10427
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>