realtek: rtl930x: add XGS1250-12 B1 device

realtek: rtl930x: add XGS1250-12 B1 device

The A1 and B1 devices are largely the same. The differences
seem to be:
- RTL8218D (A1) vs RTL8218E (B1) PHY for the eight 1 Gbps TP ports
- Aquantia (A1) vs RTL8261N (B1) PHY for the three 10 Gbps TP ports

RTL8218D/E share the same driver and support was added already by
commit c8c187f0f0 ("realtek: add support for RTL8218E").

The RTL8261N is also already supported but it's located at
different addresses compared to the A1 device. This requires
the device tree to be split. As a result, the devices are require
different images.

I found the smi addresses on the forum:
https://forum.openwrt.org/t/support-for-rtl838x-based-managed-switches/57875/3622
And I can conform on my B1 device that this is working.

Co-developed-by: Mathias Kresin <[email protected]>
Signed-off-by: Thomas Martitz <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20150
Signed-off-by: Hauke Mehrtens <[email protected]>