uboot-ath79: add support for NEC Aterm series based on AR9344

uboot-ath79: add support for NEC Aterm series based on AR9344

Add support for NEC Aterm series devices based on Atheros AR9344.
The following devices have almost the same hardware, so the same U-Boot
binary can be used for them.

- NEC Aterm WR8750N
- NEC Aterm WR9500N
- NEC Aterm WG600HP

By the way, on NetBSD-based NEC Aterm devices, only 0x20000 (128KiB) is
available for a bootloader on the flash chip and that limitation is too
small for mainline U-Boot with the default options. So many
features/commands not required for booting OpenWrt and recoverying are
disabled on that devices, like the followings.

- networking support
- FIT support
- all decompression methods support

etc...

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/15432
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>