acpi: apei: check for pending errors when probing GHES entries

Check for pending errors when probing GHES entries. It is possible
that a fatal error is already pending at this point, so we should
handle it as soon as the driver is probed. This also avoids a
potential issue if there was an interrupt that was already
cleared for an error since the GHES driver wasn't present.

Signed-off-by: Tyler Baicar <tbaicar@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>