net: ethernet: mtk_eth_soc: ppe: add support for flow accounting

The PPE units found in MT7622 and newer support packet and byte
accounting of hw-offloaded flows. Add support for reading those counters
as found in MediaTek's SDK[1].

[1]: https://git01.mediatek.com/plugins/gitiles/openwrt/feeds/mtk-openwrt-feeds/+/bc6a6a375c800dc2b80e1a325a2c732d1737df92


Tested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>