powerpc: Context switch the new EBB SPRs

This context switches the new Event Based Branching (EBB) SPRs.  The three new
SPRs are:
  - Event Based Branch Handler Register (EBBHR)
  - Event Based Branch Return Register (EBBRR)
  - Branch Event Status and Control Register (BESCR)

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/processor.h

@@ -281,6 +281,9 @@ struct thread_struct {
 #endif
 #ifdef CONFIG_PPC_BOOK3S_64
 	unsigned long	tar;
+	unsigned long	ebbrr;
+	unsigned long	ebbhr;
+	unsigned long	bescr;
 #endif
 };