net: dsa: rzn1-a5psw: add statistics support

Add statistics support to the rzn1-a5psw driver by implementing the
following dsa_switch_ops callbacks:
- get_sset_count()
- get_strings()
- get_ethtool_stats()
- get_eth_mac_stats()
- get_eth_ctrl_stats()
- get_rmon_stats()

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>