PM / devfreq: memlat: Aggregate memlat mons under a controller

Currently, each individual arm-memlat-mon instance separately reads from
a few shared PMUs, which leads to unnecessary IPI overhead. Tweak the
arm-memlat-mon design to allow multiple monitors to share certain event
counts, lowering overhead.

Change-Id: Ic3358afd1853efd84a4c3d5f3d66c3a86df6e5e7
Signed-off-by: Jonathan Avila <avilaj@codeaurora.org>
Signed-off-by: Amir Vajid <avajid@codeaurora.org>