mem-cache: Refactor the cache recvTimingResp function

mem-cache: Refactor the cache recvTimingResp function

The recvTimingResp function in the cache handles timing
responses. Over time, recvTimingResp has grown in complexity and code
size. This change factors out some of its functionality to a separate
function. The new function iterates through the in-service targets and
handles them accordingly.

Change-Id: I0ef28288640f6be1b30452b0664d32432e692ea6
Reviewed-on: https://gem5-review.googlesource.com/10423
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>