realtek: Rename ZyXEL XGS1210-12 to XGS1210-12 a1

realtek: Rename ZyXEL XGS1210-12 to XGS1210-12 a1

A new version of the ZyXEL XGS1210-12 has been discovered in
the wild. It includes at least two known hardware changes

- lan9/lan10 use RTL8221B instead of RTL8226
- lan9/lan10 use different SMI busses

Pave the new device the way by splitting the existing DTS.
According to the vendor website the models are named

- A1 (first version): not explicetly labeled
- B1 (second version): Label Rev. B1 on device

Rename the current OpenWrt device definition to A1 as it was
made for the first version. To stay compatible with older
installations, add the old device name to the list of
supported devices.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/19908
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>