refactor(qemu-sbsa): rename function sip_svc_init() to something more meaningful

There is no relation between the name of function sip_svc_init() and
what it does.  As such rename it to something more appropriate and move
it to a header that make sense.

No change in functionality.

Change-Id: I7bd78b1fe70e2930c395ef0a097bfad3b1e55d3a
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
4 files changed