Add book3s_64 highmem asm code

This is the of entry / exit code. In order to switch between host and guest
context, we need to switch register state and call the exit code handler on
exit.

This assembly file does exactly that. To finally enter the guest it calls
into book3s_64_slb.S. On exit it gets jumped at from book3s_64_slb.S too.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>