powerpc: Add dispatch trace log fields to lppaca

PAPR v2.3 defines fields in the virtual processor area for a dispatch
trace log (DLT). Since we'd like to use the DLT, add the necessary
fields to struct lppaca.

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