ipq806x: remove "firmware" partition definition from netgear routers

ipq806x: remove "firmware" partition definition from netgear routers

Remove the "firmware" partition definition from the DTS of R7800
to fix sysupgrade.

Commit 4645a6d3 defined CONFIG_MTD_SPLIT_UIMAGE_FW=y for ipq806x
and that causes mtd to misbehave as additional kernel and ubi
partitions are detected from inside the "firmware" partition.
  [    1.111324] 0x000001480000-0x000001880000 : "kernel"
  [    1.121005] 0x000001880000-0x000007900000 : "ubi"
  [    1.283912] 0x000007900000-0x000008000000 : "reserve"
  [    1.296407] 0x000001480000-0x000007900000 : "firmware"
  [    1.468043] no rootfs found after FIT image in "firmware"
  [    2.426860] 2 uimage-fw partitions found on MTD device firmware
  [    2.426931] 0x000001480000-0x000001880000 : "kernel"
  [    2.440420] 0x000001880000-0x000007900000 : "ubi"

Both kernel and ubi are already defined in DTS, so this duplication
leads into errors in sysupgrade:
  Writing from <stdin> to kernel ...
  ubiattach: error!: strtoul: unable to parse the number '6 mtd10'
  ubiattach: error!: bad MTD device number: "6 mtd10"

The partition is defined to same area as kernel+ubi, and is not
needed for sysupgrade anymore. Remove it to fix things.

Only tested for the R7800 but all of them should behave equal.

Fixes: FS#1617

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
[squashed commits, add "tested on" note]
Signed-off-by: Mathias Kresin <dev@kresin.me>