firmware: Break out system_event_log in dmi-sysfs

The optional type 15 entry of the DMI table describes a non-volatile
storage-backed system event log.

In preparation for the next commit which exposes the raw bits of the
event log to userland, create a new sub-directory within the dmi entry
called "system_event_log" and expose attribute files that describe the
event log itself.

Currently, only a single child object is permitted within a
dmi_sysfs_entry.  We simply point at this child from the dmi_sysfs_entry
if it exists.

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