ACPI: create _SUN sysfs file

_SUN method provides the slot unique-ID in the ACPI namespace. And The value
is written in Advanced Configuration and Power Interface Specification as
follows:

"The _SUN value is required to be unique among the slots ofthe same type.
It is also recommended that this number match the slot number printed on
the physical slot whenever possible."

So if we can know the value, we can identify the physical position of the
slot in the system.

The patch creates "sun" file in sysfs for identifying physical position
of the slot.

Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Reviewed-by: Toshi Kani <toshi.kani@hp.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>