ACPI: Add new sysfs interface to export device description

Add support to export the device description obtained from the ACPI _STR
method, if one exists for a device, to user-space via a sysfs interface.
This new interface provides a standard and platform neutral way for users
to obtain the description text stored in the ACPI _STR method.  If no
_STR method exists for the device, no sysfs 'description' file will be
created.  The 'description' file will be located in the /sys/devices/
directory using the device's path.

/sys/device/<bus>/<bridge path>/<device path>.../firmware_node/description

Example:

/sys/devices/pci0000:00/0000:00.07.0/0000:0e:00.0/firmware_node/description

It can also be located using the ACPI device path, for example:

/sys/devices/LNXSYSTM:00/device:00/ACPI0004:00/PNP0A08:00/device:13/device:15/description
/sys/devices/LNXSYSTM:00/device:00/ACPI0004:00/ACPI0004:01/ACPI0007:02/description

Execute the 'cat' command on the 'description' file to obtain the
description string for that device.

This patch also includes documentation describing how the new sysfs
interface works

Changes from v1-v2 based on comments by Len Brown and Fengguang Wu
* Removed output "No Description" and leaving a NULL attribute if the
_STR method failed to evaluate.

* In acpi_device_remove_files() removed the redundent check of
dev->pnp.str_obj before calling free.  This check triggered a message
from smatch.

Signed-off-by: Lance Ortiz <lance.ortiz@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>