generic: provide get_port_stats() on ar8xxx switches

This patch provides a generic switch_dev_ops 'get_port_stats()' callback by
taping into the relevant port MIB counters.

The implementation uses a generic callback that select the correct MIB counter
index based on chip version.

This callback is used by swconfig_leds led trigger to blink LEDs with port
network traffic.

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>