mtd: add Broadcom BCM63xx image tag partition parser

This patch adds support for parsing Broadcom BCM63xx image tag format and
creating MTD partitions accordingly. This driver is a platform_device which
can be instantiated accordingly by bcm63xx board support code.

Signed-off-by: Daniel Dickinson <cshore@csolve.net>
Signed-off-by: Mike Albon <malbon@openwrt.org>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>