powerpc/pseries: Reduce HVCS driver insanity

The HVCS driver, for those who don't know, is a driver for the "server" side
of the IBM virtual terminal mechanism allowing Linux partitions to act as
terminal servers under IBM PowerVM hypervisor. It's almost never used on
the field at the moment.

However, it's part of our configs, and in its current incarnation, will
allocate the tty driver & major (with 64 minors) and create a kernel thread
whether it's used or not, ie, whether the hypervisor did put a virtual
terminal server device node in the partition or not (or whether running on
a pseries machine or not even).

This in turns causes modern distro's udev's to start trying to open all
those 64 minors at boot, which, since they aren't linked to anything,
causes the driver to spew errors in the kernel log for each of them.

Not nice.

This moves all that initialization to a function which is now only called
the first time a terminal server virtual IO device is actually probed
(that is almost never).

There's still a _LOT_ of cleanup that can be done in this driver, some
simple (almost all printk's statements in there shall either just be
removed or in some case turned into better written & more informative
messages, including using the dev_* variants etc...). This is left as
an exercise for whoever actually cares about that driver.

One could also try to be smart and dispose of all the tty related
resources when the last instance of the VIO server device
is removed (Hotplug anybody ?).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>