uboot-ath79: add support for NEC Aterm series based on QCA9558

uboot-ath79: add support for NEC Aterm series based on QCA9558

Add support for NEC Aterm series devices based on QCA9558.
The following devices have almost the same hardware, so the same U-Boot
binary can be used for them.

- NEC Aterm WG1400HP
- NEC Aterm WG1800HP
- NEC Aterm WG1800HP2

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/16297
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>