ARM: dts: Make vcio a child of the firmware node

In order to resolve a potential startup order bug, the vcio driver has
been rewritten as a platform driver that depends on a DT node for
its instantiation and to locate the firmware driver.

Add that DT node.

See: https://github.com/raspberrypi/linux/issues/4620



Signed-off-by: Phil Elwell <phil@raspberrypi.com>