mem: Allow inserts in the begining of a packet queue

mem: Allow inserts in the begining of a packet queue

A packet queue keeps track of packets that are scheduled to be sent at
a specified time. Packets are sorted such that the packet with the
earliest scheduled time is at the front of the list (unless there are
other ordering requirements). Previouly, the implemented algorithm
didn't allow packets to be placed at the front of the queue resulting
in uneccessary delays. This change fixes the implementation of
schedSendTiming.

Change-Id: Ic74abec7c3f4c12dbf67b5ab26a8d4232e18e19e
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15556
Reviewed-by: Bradley Wang <radwang@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>