ath79: add Cisco Meraki MR18

ath79: add Cisco Meraki MR18

Specifications:

SOC:    Atheros/Qualcomm QCA9557-AT4A @ 720MHz
RAM:    2x Winbond W9751G6KB-25 (128 MiB)
FLASH:  Hynix H27U1G8F2BTR-BC TSOP48 ONFI NAND (128 MiB)
WIFI1:  Atheros AR9550 5.0GHz (SoC)
WIFI2:  Atheros AR9582-AR1A 2.4GHz
WIFI2:  Atheros AR9582-AR1A 2.4GHz + 5GHz
PHYETH: Atheros AR8035-A, 802.3af PoE capable Atheros (1x Gigabit LAN)
LED:    1x Power-LED, 1 x RGB Tricolor-LED
INPUT:  One Reset Button
UART:   JP1 on PCB (Labeled UART), 3.3v-Level, 115200n8
        (VCC, RX, TX, GND - VCC is closest to the boot set jumper
	 under the console pins.)

Flashing instructions:

Depending on the installed firmware, there are vastly different
methods to flash a MR18. These have been documented on:
<https://openwrt.org/toh/meraki/mr18>

Tip:
Use an initramfs from a previous release and then use sysupgrade
to get to the later releases. This is because the initramfs can
no longer be built by the build-bots due to its size (>8 MiB).

Note on that:
Upgrades from AR71XX releases are possible, but they will
require the force sysupgrade option ( -F ).

Please backup your MR18's configuration before starting the
update. The reason here is that a lot of development happend
since AR71XX got removed, so I do advise to use the ( -n )
option for sysupgrade as well. This will cause the device
to drop the old AR71xx configuration and make a new
configurations from scratch.

Note on LEDs:
The LEDs has changed since AR71XX. The white LED is now used during
the boot and when upgrading instead of the green tricolor LED. The
technical reason is that currently the RGB-LED is brought up later
by a userspace daemon.

(added warning note about odm-caldata partition. remove initramfs -
it's too big to be built by the bots. MerakiNAND -> meraki-header.
sort nu801's targets)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>