ARM: socfpga: arria10: Add ptp_ref clock to ethernet nodes

The ptp_ref clock for Arria10 defaults to using the peripheral
pll emac ptp clock.  Without the ptp_ref clock in the gmac nodes
the driver defaults to the gmac main clock resulting in an
incorrect period for the ptp counter.

Signed-off-by: Dalon Westergreen <dalon.westergreen@linux.intel.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>