apm821xx: Add support for the Cisco Meraki MX60/MX60W

apm821xx: Add support for the Cisco Meraki MX60/MX60W

This patch adds support for the Cisco Meraki MX60/MX60W Security
Appliance. Flashing information can be found at
https://github.com/riptidewave93/LEDE-MX60

Specs are as follows:
AppliedMicro APM82181 SoC at 800MHz
1GiB NAND - Samsung K9K8G08U0D
512MB DDR RAM - 4x Nanya NT5TU128M8GE-AC
Atheros AR8327-BL1A Gigabit Ethernet Switch
1x USB 2.0 Port

More info can be found at https://wiki.openwrt.org/toh/meraki/mx60

Cc: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Chris Blake <chrisrblake93@gmail.com>