net: dsa: hellcreek: Add PTP status LEDs

The switch has two controllable I/Os which are usually connected to LEDs. This
is useful to immediately visually see the PTP status.

These provide two signals:

 * is_gm

   This LED can be activated if the current device is the grand master in that
   PTP domain.

 * sync_good

   This LED can be activated if the current device is in sync with the network
   time.

Expose these via the LED framework to be controlled via user space
e.g. linuxptp.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>