msm: SSR: Add subsystem_get()/subsystem_put() API

In an effort to merge SSR and PIL frameworks, introduce a get/put
API into SSR to eventually replace the get/put API in PIL. Once
all PIL drivers are hooked into the subsystem restart framework
we will do a tree-wide sed to replace pil_get() and pil_put() with
their subsystem equivalent.

Change-Id: Id2208b25b34230c7b8ed55c126aee6491f77441e
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>