char: vcio: Rewrite as a firmware node child

The old vcio driver is a simple character device that manually locates
the firmware driver. Initialising it before the firmware driver causes
a failure, and no retries are attempted.

Rewrite vcio as a platform driver that depends on a DT node for its
instantiation and the location of the firmware driver, making use of
the miscdevice framework to reduce the code size.

N.B. Using miscdevice changes the udev SUBSYSTEM string, so a change
to the companion udev rule is required in order to continue to set
the correct device permissions, e.g.:

    KERNEL="vcio", GROUP="video", MODE="0660"

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



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