ANDROID: GKI: bfq-iosched: update struct elevator_mq_ops ABI

Add the function pointer elevator_registered_fn to the struct
elevator_mq_ops to allow vendors use of this function.

Bug: 152659776
Test: compile
Change-Id: I9e55eee03917a1ab07fbd3f04635ca1a6541b860
Signed-off-by: Pradeep P V K <ppvk@codeaurora.org>
(cherry picked from commit da1defc781fdbf66a12ff8dbb1926d5fc507521e)
[willmcvicker: only cherry-picked the ABI diff]
Signed-off-by: Will McVicker <willmcvicker@google.com>