ramips: ethernet: ralink: add mediatek,no-swconfig property

ramips: ethernet: ralink: add mediatek,no-swconfig property

Skip swconfig registration and disable MAC learning when the
"mediatek,no-swconfig" property is set. This is needed for MT7620
devices using the internal switch as a transparent bridge to an
external RGMII switch, where ports 0-4 are unused.

The no-swconfig property itself is valid for both DSA and non-DSA
setups. However, disabling MAC learning is only required for DSA,
where multiple CPU-facing interfaces would otherwise cause an
incorrect ARL table to be built. Non-DSA setups work correctly
without disabling MAC learning.

Signed-off-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Link: https://github.com/openwrt/openwrt/pull/23933
Signed-off-by: Robert Marko <robimarko@gmail.com>