apm821xx: increase WNDR4700's dtb+kernel partition to 3.5 MiB

apm821xx: increase WNDR4700's dtb+kernel partition to 3.5 MiB

This patch cleans and reworks the WNDR4700 dts to increase the
now combined dtb+kernel partition to 3.5 MiB. This has become
necessary due to the switch to GCC 7.3 and the ever increasing
kernel binary size.

The dtb+kernel partition was combined in order to finally
fix the problem with out-of-sync device-trees. From now
on, the kernel and device-tree will always be updated together.

Upgrade Note:
Existing installations will have to use the TFTP firmware
recovery option in order to install the update. Affected users
are advised to make a backup of their existing configuration
prior to running sysupgrade:
<https://openwrt.org/docs/guide-user/installation/generic.backup#backup_openwrt_configuration>
Due to the repartitioning of the NAND, the generated backup
should be placed on either the internal HDD, an attached
USB-Stick or on another PC (externally).

To manually trigger the firmware recovery, the reset button has
to be pressed (and hold) during boot. U-boot will enter the "Upgrade
Mode" and starts a tftpserver listening on 192.168.1.1 for a
tftp client from one of the four LAN/Ethernet ports to connect and
upload the new system: (enable tftp binary mode!).
openwrt-apm821xx-nand-netgear_wndr4700-squashfs-factory.img

Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>