realtek: create central DTS macro include

realtek: create central DTS macro include

The Realtek DTS's use several macros for convenient phy/port definition.
These are repeated for the RTL83xx targets and most are missing for the
RTL93xx targets. In the near future we want to add high port count
switches with 1GBit Ethernet for them too. As a preparation provide a
central include so the definition is only needed once and is available
for all targets.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/19772
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>