mem-cache: Add getter for the number of valid sub-blks

mem-cache: Add getter for the number of valid sub-blks

Add a getter function so that the number of valid sub-blocks can be
retrieved. As a side effect, make the respective counter private.

Change-Id: Icef8b51164c8e165872dcaebc65f5c330f16cb29
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22605
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>