realtek: rtl838x: rename GS1900 series v1/v2 to A1/B1

realtek: rtl838x: rename GS1900 series v1/v2 to A1/B1

Zyxel labels their switch revisions A1, B1, ... and not v1, v2, ...
Rename the devices as such in OpenWrt to match the labels. Of note:
the first (A1) revision is never labeled as such on the label, just
in the web UI. Provide compatibles for seamless sysupgrade.

For a recent overview of Zyxel GS1900 series revisions, see the
table linked in https://forum.openwrt.org/t//57875/3874.

Signed-off-by: Stijn Segers <foss@volatilesystems.org>
Link: https://github.com/openwrt/openwrt/pull/20118
Signed-off-by: Robert Marko <robimarko@gmail.com>