ptp: restore the makefile for building the test program.

This patch brings back the makefile called testptp.mk which was removed
in commit adb19fb6 (Documentation: add makefiles for more targets).

While the idea of that commit was to improve build coverage of the
examples, the new Makefile is unable to cross compile the testptp program.
In contrast, the deleted makefile was able to do this just fine.

This patch fixes the regression by restoring the original makefile.
Signed-off-by: NRichard Cochran <richardcochran@gmail.com>
Acked-by: NPeter Foley <pefoley2@pefoley.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/ptp/testptp.mk

+# PTP 1588 clock support - User space test program
+#
+# Copyright (C) 2010 OMICRON electronics GmbH
+#
+#  This program is free software; you can redistribute it and/or modify
+#  it under the terms of the GNU General Public License as published by
+#  the Free Software Foundation; either version 2 of the License, or
+#  (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with this program; if not, write to the Free Software
+#  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+
+CC        = $(CROSS_COMPILE)gcc
+INC       = -I$(KBUILD_OUTPUT)/usr/include
+CFLAGS    = -Wall $(INC)
+LDLIBS    = -lrt
+PROGS     = testptp
+
+all: $(PROGS)
+
+testptp: testptp.o
+
+clean:
+	rm -f testptp.o
+
+distclean: clean
+	rm -f $(PROGS)