powerpc/perf/hv_{gpci, 24x7}: Add documentation of device attributes

gpci and 24x7 expose some device specific attributes. Add some
documentation for them.

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>