soc: qcom: smp2p: Add memory barrier for irq_pending

There is a very tight race where the irq_retrigger function is run
on one cpu and the actual retrigger softirq is running on a second
cpu. When this happens, there may be a chance that the second cpu
will not see the updated irq_pending value from first cpu.

Add a memory barrier to ensure that irq_pending is read correctly.

Change-Id: I3dd185decc4f050bd57c0b6558f417ead2a3aa5a
Signed-off-by: Chris Lew <clew@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>