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: Richard Cochran <richardcochran@gmail.com>
Acked-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: David S. Miller <davem@davemloft.net>