msm: pil-8660: Break off modem code into platform driver

Create a platform driver to manage the modem instead of having a
module that exists for any 8660 device. This allows us to
add platform devices when the modem really exists and not add it
when there isn't a modem preset (e.g. APQ devices).

Change-Id: I7940c6430ded009dd8ea2cae9ffa514f5495d3f3
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>