hvc_console: Export (GPL'ed) hvc_remove

The virtio console, which uses hvc, will get the ability to hot-unplug
ports. Export hvc_remove so that virtio_console can disassociate with
hvc.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Cc: linuxppc-dev@ozlabs.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>