realtek: restructure Zyxel XGS1210-12 device tree files

realtek: restructure Zyxel XGS1210-12 device tree files

This is a preparation for adding support for XGS1010-12, which is almost
identical to XGS1210-12, with some small differences (partition layout,
missing reset key).

In addition to moving the common parts to a new file, also simplify the
definition of the 2.5G PHYs to reduce duplication. With this change, the
revision-specific files only have to specify the SMI addresses.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>
Link: https://github.com/openwrt/openwrt/pull/20469
Signed-off-by: Robert Marko <robimarko@gmail.com>