ath79: add GL.iNet GL-AR300M16 as NOR-only board

ath79: add GL.iNet GL-AR300M16 as NOR-only board

The GL.iNet GL-AR300M series of devices includes variants without NAND
and only the 16 MB NOR flash. These include the GL-AR300M16 and the
GL-AR300M-Lite (already with its own board name).

This board-name addition provides disambiguation from the NAND-bearing
GL-AR300M devices, both for OpenWrt code and for end users.

Kernel and firmware support for NAND and UBI will add ~320 kB to the
overall firmware size at this time. This NOR-only option continues to
provide more compact firmware for both the GL-AR300M16 as well as
those who wish to use it as an alternate or primary, NOR-resident
firmware on the GL-AR300M.

The ar71xx targets are unmodified.

Installation
------------

Install through OEM U-Boot (HTTP-based) or `sysupgrade --force` when
booted from NOR and running OEM or OpenWrt, NOR-based firmware.

As one of the intentions is disambiguation from NAND-bearing units,
users who have flashed this firmware onto a device with NAND would
need to use U-Boot or `sysupgrade --force` to flash firmware that
again supports NAND.

There are no additional SUPPORTED_DEVICES as it is not possible to
determine if a device does or does not have NAND based on
either the OEM's or OpenWrt's board names prior to this patch.

Signed-off-by: Jeff Kletsky <git-commits@allycomm.com>