[SRU][Zesty] acpi: apei: check for pending errors when probing GHES entries

[Manoj Iyer]

Please review and consider the following patch to bug 
https://launchpad.net/bugs/1698448. This patch was cleanly cherry-picked from
ARM64 git repo for-next maintained by Will Deacon. 

This patch is an addition to the RAS patches I had previously submitted for SRU.
Without this patch pending RAS error at boot may not be handled correctly on
the QDF2400 platforms, and could cause platform to reboot, or prevent future
future RAS issues from being reported.

I have a test kernel (ARM64/AMD64/Power8) in PPA: 
https://launchpad.net/~centriq-team/+archive/ubuntu/lp1698448/.
Since testing the kernel requires injecting an error before kernel boot, the
testing was done by QTI and reported results back in the bug. I was able to
boot test the kernel on QDF2400 server at Canonical.