input: joystick: Add support for ShinEtsu VTD518 Touchdisc

VTD518 Touchdisc is a device from ShinEtsu supporting communication
over I2C. It is a multifunction touchdisc which supports rotary
scrolling, X/Y absolute positioning, X/Y relative movement,
kinetic scrolling and 5-way switch input.

The device reports various motion and key-press events via
interrupts. Based on the interrupt status, the driver communicates
with the device over I2c and receives the events. These events
are then reported to the input subsystem.

CRs-Fixed: 237218
Change-Id: Ib40bf4a6e294eae4b0199101a1725c226139da71
Signed-off-by: Anirudh Ghayal <aghayal@qualcomm.com>