ARM: dts: bcm2835: make the firmware node into a bus

This allows adding devices for which the firmware exposes control interface
via the mailbox. An example of such device is the GPIO expander.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>