ath79: GL-AR300M: provide NAND support; increase to 4 MB kernel

ath79: GL-AR300M: provide NAND support; increase to 4 MB kernel

The GL.iNet GL-AR300M has been supported by the ar71xx and ath79
platforms with access to its 16 MB NOR flash, but not its 128 MB
SPI NAND flash.

This commit provides support for the NAND through the upstream
SPI-NAND framework. Devices with both NOR and NAND flash can support
independent firmware on each, with U-Boot able to boot from either.
The OEM U-Boot will fall back to the NOR firmware after three
"unsuccessful" boots.

The family of GL-AR300M devices on the ath79 platform now includes:

  * glinet,gl-ar300m-lite       "generic" target, NOR-only board
  * glinet,gl-ar300m-nand       "nand" target
  * glinet,gl-ar300m-nor        "nand" target (NAND-aware)

NB: This commit increases the kernel size from 2 MB to 4 MB

"Force-less" sysupgrade is presently supported from the current
versions of following NOR-based firmwre images to the version of
glinet,gl-ar300m-nor firmware produced by this commit:

  * gl-ar300m            -- OEM v3 NOR    ar71xx (openwrt-ar300m16-*.bin)
  * gl-ar300m            -- OpenWrt 18.06 ar71xx
  * gl-ar300m            -- OpenWrt 19.07 ar71xx

Other upgrades to these images should be performed through U-Boot.

The GL-AR300M OEM U-Boot allows upload and flashing of either NOR
firmware (sysupgrade.bin) or NAND firmware (factory.img) through its
HTTP-based GUI. Serial connectivity is not required.

The glinet,gl-ar300m-nand and glinet,gl-ar300m-nor images generated
after this commit should safely flash each other using sysupgrade.

The boot counter is implemented by the OEM using u-boot-env. At this
time, it does not appear that the switch on the side of the unit can
be used to select NOR vs. NAND boot and the fail-over is only from
NAND to NOR. To save flash wear, it is only reset when running the
glinet,gl-ar300m-nand firmware.

NAND-specific base-files are used to remove impact on existing
generic and tiny targets.

As there is now no "generic" build appropriate for the GL-AR300M16,
(or for users of the GL-AR300M that do not need access to NAND)
it will be introduced in a subsequent commit.

Note: `mtd_get_mac_binary art 0x6` does not return the proper MAC
and the GL.iNet source indicates that only the 0x0 offset is valid

The ar71xx targets are unmodified.

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