firmware-utils: Add support for the Cisco Meraki MX60/MX60W

firmware-utils: Add support for the Cisco Meraki MX60/MX60W

This patch adds header support for the Cisco Meraki MX60/MX60W, which
are a part of the apm821xx target. Some structure changes were needed
due to the fact this device uses U-Boot (unlike other devices in
mkmerakifw.c) which uses a different header structure to define the load
offsets for the image.

A thanks to Christian for helping implement this properly.

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