Feedback from Hidetoshi Seto and Kenji Kaneshige incorporated.  This
correctly handles PCI-X bridges, PCIe root ports and endpoints, and
prints debug messages when invalid/reserved types are found in the
HEST.  PCI devices not in domain/segment 0 are not represented in
HEST, thus will be ignored.

Today, the PCIe Advanced Error Reporting (AER) driver attaches itself
to every PCIe root port for which BIOS reports it should, via ACPI
_OSC.

However, _OSC alone is insufficient for newer BIOSes.  Part of ACPI
4.0 is the new APEI (ACPI Platform Error Interfaces) which is a way
for OS and BIOS to handshake over which errors for which components
each will handle.  One table in ACPI 4.0 is the Hardware Error Source
Table (HEST), where BIOS can define that errors for certain PCIe
devices (or all devices), should be handled by BIOS ("Firmware First
mode"), rather than be handled by the OS.

Dell PowerEdge 11G server BIOS defines Firmware First mode in HEST, so
that it may manage such errors, log them to the System Event Log, and
possibly take other actions.  The aer driver should honor this, and
not attach itself to devices noted as such.

Furthermore, Kenji Kaneshige reminded us to disallow changing the AER
registers when respecting Firmware First mode.  Platform firmware is
expected to manage these, and if changes to them are allowed, it could
break that firmware's behavior.

The HEST parsing code may be replaced in the future by a more
feature-rich implementation.  This patch provides the minimum needed
to prevent breakage until that implementation is available.

Reviewed-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Reviewed-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Matt Domsch <Matt_Domsch@dell.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>