selftests: move ptp tests from Documentation/ptp

Remove ptp from Makefile to move the test to selftests. Update ptp Makefile
to work under selftests. ptp will not be run as part of selftests suite and
will not be included in install targets. They can be built separately for
now.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>