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 fastly different methods
to flash a MR18. These have been documented on:
<https://openwrt.org/toh/meraki/mr18>

Note: upgrades from AR71XX are possible, but require the force
sysupgrade option.

The LEDs has changed since AR71XX. The white LED is now used during
the boot and when upgrading instead of the green tricolor LED.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>