input: ti_am335x_tsc: Enable shared IRQ for TSC

Enable shared IRQ to allow ADC to share IRQ line from
parent MFD core. Only FIFO0 IRQs are for TSC and handled
on the TSC side.

Step mask would be updated from cached variable only previously.
In rare cases when both TSC and ADC are used, the cached
variable gets mixed up.
The step mask is written with the required mask every time.

Rachna Patil (TI) laid ground work for shared IRQ.

Signed-off-by: Zubair Lutfullah <zubair.lutfullah@gmail.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>