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: NJonathan Corbet <corbet@lwn.net>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

Documentation/Makefile

 subdir-y := accounting auxdisplay blackfin \
 	ia64 laptops mic misc-devices \
-	networking pcmcia ptp timers vDSO watchdog
+	networking pcmcia timers vDSO watchdog

Documentation/ptp/Makefile

-# List of programs to build
-hostprogs-y := testptp
-
-# Tell kbuild to always build the programs
-always := $(hostprogs-y)
-
-HOSTCFLAGS_testptp.o += -I$(objtree)/usr/include
-HOSTLOADLIBES_testptp := -lrt

tools/testing/selftests/ptp/Makefile

+TEST_PROGS := testptp
+LDLIBS += -lrt
+all: $(TEST_PROGS)
+
+include ../lib.mk
+
+clean:
+	rm -fr $(TEST_PROGS)