firmware: Expose DMI type 15 System Event Log

The System Event Log described by DMI entry type 15 may be backed by
either memory or may be indirectly accessed via an IO index/data
register pair.

In order to get read access to this log, expose it in the
"system_event_log" sub-directory of type 15 DMI entries, ie:
/sys/firmware/dmi/entries/15-0/system_event_log/raw_event_log.

This commit handles both IO accessed and memory access system event
logs.  OEM specific access and GPNV support is explicitly not handled
and we error out in the logs when we do not recognize the access method.

Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>