powerpc: Add virtual processor dispatch trace log

pseries SPLPAR machines are able to retrieve a log of dispatch and
preempt events from the hypervisor. With this information, we can
see when and why each dispatch & preempt is occuring.

This change adds a set of debugfs files allowing userspace to read this
dispatch log.

Based on initial patches from Nishanth Aravamudan <nacc@us.ibm.com>.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>