soc: qcom: pil: Delay IRQ registration in subsys registration

When the request_irq functions and any of its variants
are invoked, it is possible to receive an IRQ from the
moment the IRQ line becomes active. This should only be
done when all other initialization routines have been
performed, to ensure proper behavior. Make IRQ registration
the last step in the subsystem registration process,
and initialize completion variables used in PIL IRQ handlers
to ensure that the completion variables used in these IRQ
handlers are not used prior to initialization.

Change-Id: I45286284d3c6d88c6b58dd4e2c2092f373b49fa1
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>