mem: Add function to check if the slave can receive a timing req

mem: Add function to check if the slave can receive a timing req

This changeset adds support for tryTiming, an interface that allows a
master to check if the slave is busy or otherwise if it can accept a
timing request.

Change-Id: Idc7c2337ae9ccf5dec54f308e488660591419a63
Reviewed-on: https://gem5-review.googlesource.com/5041
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Christian Menard <christian.menard@tu-dresden.de>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>