powerpc/perf: Clean up the EBB hash defines a little

Rather than using PERF_EVENT_CONFIG_EBB_SHIFT everywhere, add an
EVENT_EBB_SHIFT like every other event and use that.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>