powerpc: introduce the ePAPR embedded hypervisor vmpic driver

The Freescale ePAPR reference hypervisor provides interrupt controller
services via a hypercall interface, instead of emulating the MPIC
controller.  This is called the VMPIC.

The ePAPR "virtual interrupt controller" provides interrupt controller
services for external interrupts.  External interrupts received by a
partition can come from two sources:

  - Hardware interrupts - hardware interrupts come from external
    interrupt lines or on-chip I/O devices.
  - Virtual interrupts - virtual interrupts are generated by the hypervisor
    as part of some hypervisor service or hypervisor-created virtual device.

Both types of interrupts are processed using the same programming model and
same set of hypercalls.

Signed-off-by: Ashish Kalra <ashish.kalra@freescale.com>
Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>