PM / devfreq: bimc-bwmon: Fix counter clearing

The counter needs to be cleared and acknowledged before clearing the IRQ
bits. Otherwise, the HW could set the IRQ bit again if the old counter
value was higher than the threshold. So, add a memory barrier after
clearing the counter.

Change-Id: I35f9f7905b05b8a185eb94d04d9c0a8ccfc2db51
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>