msm: pil: Stub out PIL API if not compiled

If PIL is not enabled, we must stub out functions such as
msm_pil_{register,unregister}() and scm_pas_*() so that the build
still compiles. This is especially important when we merge the
SSR driver with the PIL driver for the modem on 8960 and 9615
where 9615 doesn't have PIL enabled.

Change-Id: I56689b6243fd1e45e866591a9a5a93891a0468e4
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>