powerpc/booke64: Use GSRR registers in Guest Doorbell interrupts

Guest Doorbell interrupts use guest save and restore registers. Add a new
Guest Doorbell exception type to accommodate GSRR0/1 SPRs usage in exception
prolog and fix the exception handler.

Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>