xen: Add files needed for minimal ppc64le build

Add the build system changes required to build for ppc64le (POWER8+).
As of now the resulting image simply boots to an infinite loop.

$ make XEN_TARGET_ARCH=ppc64 -C xen build

This port targets POWER8+ CPUs running in Little Endian mode specifically,
and does not boot on older machines. Additionally, this initial skeleton
only implements the PaPR/pseries boot protocol which allows it to be
booted in a standard QEMU virtual machine:

$ qemu-system-ppc64 -M pseries-5.2 -m 256M -kernel xen/xen

Signed-off-by: Shawn Anastasio <sanastasio@raptorengineering.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

config/ppc64.mk

@@ -0,0 +1,4 @@
+CONFIG_PPC := y
+CONFIG_PPC64 := y
+
+CONFIG_XEN_INSTALL_SUFFIX :=