powerpc/perf: Export PERF_EVENT_CONFIG_EBB_SHIFT to userspace

We use bit 63 of the event code for userspace to request that the event
be counted using EBB (Event Based Branches). Export this value, making
it part of the API - though only on processors that support EBB.

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