realtek: add 6.1 as testing kernel

realtek: add 6.1 as testing kernel

Kernel 6.1 successfully compiles and boots on devices from all
subtargets (Zyxel GS1900-8, Zyxel GS1900-48, Zyxel XGS1250-12).

Although not extensively tested yet, rtl838x and rtl839x appear to
function correctly.

The network bridge device on rtl930x is not fully functional yet.
Traffic can be received by the kernel, as the ARP table receives updates
on external pings. However, traffic is then lost somehow, and it appears
to be only sporadically possible to obtain a properly functioning link.

Signed-off-by: Sander Vanheule <sander@svanheule.net>