realtek: Split Zyxel GS1900-8 into v1 and v2

realtek: Split Zyxel GS1900-8 into v1 and v2

Zyxel GS1900-8 v2 devices have been produced more recently than v1
devices. As there are v1 boards with RTL8380M rev. C SoCs, it can likely
safely be assumed that all v2 devices will also have a recent SoC
revision, supporting the hardware auxiliary MDIO controller.

Make the GS1900-8 v1 use an emulated auxiliary MDIO bus, for backward
compatibility with devices containing an RTL8380M rev. A.

Since the devicetrees are otherwise identical, GS1900-8 v1 devices with
an RTL8380M rev. B or C will also be able to use the (more efficient) v2
image. This includes any currently functioning device with OpenWrt, so
include the old compatible as a supported device for the GS1900-8 v2.

Link: https://github.com/openwrt/openwrt/issues/9534
Signed-off-by: Sander Vanheule <sander@svanheule.net>