ramips: mt7621: support openwrt,netdev-name for renaming interfaces

ramips: mt7621: support openwrt,netdev-name for renaming interfaces

Edgerouter X currently has its eth1 port on the switch missing since there
is a naming conflict currently.

So, as the root cause is mixing kernel support for DSA interfaces having
predictable names set via "label" property vs others having it assigned
dynamically lets avoid the conflict by using our own custom property as
suggested upstream [1].

So, add support via "openwrt,netdev-name" property and use it on ERX.

Fixes: 2a25c6ace8d8 ("ramips: get rid of downstream network device label patch")
Fixes: #15643
Link: https://github.com/openwrt/openwrt/pull/17062
Signed-off-by: Robert Marko <robimarko@gmail.com>