MFD: ucb1x00-ts: provide input layer with device parent

Provide the input layer struct device with its parent device, so
that the input layer's device appears in the correct place in the
device tree.  This also allows the input device to be visibily
associated with its hardware.

Acked-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>