UPSTREAM: PM / sleep: Add support for read-only sysfs attributes

Some sysfs attributes in /sys/power/ should really be read-only,
so add support for that, convert those attributes to read-only
and drop the stub .show() routines from them.

Original-by: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit a1e9ca69)
Signed-off-by: Alex Shi <alex.shi@linaro.org>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>