msm: pil: Make register code into a bus

Make struct pil_device into a wrapper around struct device so
that we can group all pil devices in the system onto one logical
pil bus. This will allow us to make 'online' sysfs nodes that
userspace can poll on in a later patch. Also, add an unregister
function so that devices can be removed if a module is removed.

Change-Id: I5e6c4286e3fb2149bdf2311fe23588364349eb54
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>