cpts: introduce time stamping code and a PTP hardware clock.

This patch adds a driver for the CPTS that offers time
stamping and a PTP hardware clock. Because some of the
CPTS hardware variants (like the am335x) do not support
frequency adjustment, we have implemented this in software
by changing the multiplication factor of the timecounter.

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