selftests: Add test to verify power supply properties

Add a kselftest that verifies power supply properties from sysfs and
uevent. It checks whether they are present, readable and return valid
values.

This initial set of properties is not comprehensive, but rather the ones
that I was able to validate locally.

Co-developed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>