powerpc/perf: Set PPC_FEATURE2_EBB when we register the power8 PMU

The presence or absence of EBB is advertised to userspace via the presence
or absence of PPC_FEATURE2_EBB in cpu_user_features2.

Because the kernel can be built without PMU support, we should only add
PPC_FEATURE2_EBB to cpu_user_features2 when we successfully register the
power8 PMU support.

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