char: broadcom: Add vcio module

Add module for accessing the mailbox property channel through
/dev/vcio. Was previously in bcm2708-vcio.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>

char: vcio: Add compat ioctl handling

There was no compat ioctl handler, so 32 bit userspace on a
64 bit kernel failed as IOCTL_MBOX_PROPERTY used the size
of char*.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>

char: vcio: Fail probe if rpi_firmware is not found.

Device Tree is now the only supported config mechanism, therefore
uncomment the block of code that fails the probe if the
firmware node can't be found.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>