gemini: Drop kernel v5.15 and default to v6.1

gemini: Drop kernel v5.15 and default to v6.1

There is no point in keeping the v5.15 kernel around for Gemini,
we are maintaining the platform with a strong upstream focus and
newer is always better.

Now that OpenWrt can support pure v6.1 kernels, switch up to
v6.1 and drop v5.15 so we don't need to migrate configs and
patches for no reason.

The USB FOTG2 module handling can be simplified as a result.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>