powerpc/perf: Add kconfig option for hypervisor provided counters

The commit adds a Kconfig option which allows the hv_gpci and hv_24x7
PMUs, added in the preceeding commits, to be built.

Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>