MIPS: BCM63XX: recognize Cable Modem firmware format

Add the firmware header format which is used by Broadcom Cable Modem
SoCs such as the BCM3368 SoC. We export the bcm_hcs firmware format
structure because it is used by user-land tools to create firmware
images for these SoCs and will later be used by a corresponding MTD
parser.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: cernekee@gmail.com
Cc: jogo@openwrt.org
Patchwork: https://patchwork.linux-mips.org/patch/5496/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>