gemini: sl93512r: override SL93512R partitions for firmware

gemini: sl93512r: override SL93512R partitions for firmware

This is patch is identical in form and purpose as the IB-4220-B
patch. We switch over to a single "firmware" partition.

All reference design-based machines are now converted and we can
drop the legacy set-up code.

It turns out that the reference design also uses the flash layout
with a 3072KB kernel so augment the sysupgrade to do the right
thing also here.

Link: https://github.com/openwrt/openwrt/pull/21820
Signed-off-by: Linus Walleij <linusw@kernel.org>