Add serial device bus support to penmount serial driver.

Currently the penmount touchscreen driver registers as a serio driver, which seems not supporting device tree binding. This patch tries to support both serio and serdev in a single module.
For this purpose, the module implements module_init and module_exit instead of using module_serio_driver. The module is also modified to share input codes for both serio and serdev.

Signed-off-by: John Sung <penmount.touch@gmail.com>