soc: qcom: pil: Expose function to get subsys_device pointer

There is currently no way to access the subsys_device data
structure from an external source, which prevents access to
other necessary data structures. Expose function to find
the subsys_device data structure associated with a subsystem
descriptor, so that it can be accessed.

Change-Id: I068c859b2313e4e95347dc3f96625665411e8b96
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>