ptp: add debugfs support for ptp_qoriq

This patch is to add debugfs support for ptp_qoriq. Current debugfs
supports to control fiper1/fiper2 loopback mode. If the loopback mode
is enabled, the fiper1/fiper2 pulse is looped back into trigger1/
trigger2 input. This is very useful for validating hardware and driver
without external hardware. Below is an example to enable fiper1 loopback.

echo 1 > /sys/kernel/debug/2d10e00.ptp_clock/fiper1-loopback

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>