apm821xx: WNDAP620 + WNDAP660: reorganize partitions for 5.10

apm821xx: WNDAP620 + WNDAP660: reorganize partitions for 5.10

Due to 5.10 increased kernel size, the current 4MiB-ish kernel
partition got too small. Luckily, netgear's uboot environment
is setup to read 0x60000 bytes from the kernel partition location.

... While at it: also do some cleanups in the DTS in there.

The original (re-)installation described in
commit d82d84694e60 ("apm821xx: add support for the Netgear WNDAP620 and WNDAP660")
seemed to be still working for now. What I noticed though
is that the bigger initramfs images needed to use a different
destination address (1000000) to prevent it overwriting
itself during decompression. i.e:
 # tftp 1000000 openwrt-...-wndap620-initramfs-kernel.bin
 # bootm

However, in case of the WNDAP620+660 the factory.img image can be
written directly to the flash through uboot.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>