ARM: brcmstb: add misc. DT bindings for brcmstb

Document the bindings that the Broadcom STB platform needs
for proper bootup.

Signed-off-by: Marc Carino <marc.ceeeee@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Matt Porter <mporter@linaro.org>