MAINTAINERS: add entry for the Broadcom BCM63xx ARM SoCs

Add a MAINTAINERS entry covering all the Broadcom BCM63xx ARM DSL SoCs
files along with the relevant git tree and mailing-list.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>