mem: Add Request::isMasked to check for byte strobing

mem: Add Request::isMasked to check for byte strobing

This is trying to overcome the following problem: At the moment a memory
request with a non empty byteEnable mask will be considered masking even
if all elements in the vector are true.

Change-Id: I16ae2c0ea8c3f3370e397bab9d79d6d60c3784bd
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23284
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>