PPC: Split context init/destroy functions

We need to reserve a context from KVM to make sure we have our own
segment space. While we did that split for Book3S_64 already, 32 bit
is still outstanding.

So let's split it now.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Avi Kivity <avi@redhat.com>