input: TSC: ti_tscadc: Rename the existing touchscreen driver

Make way for addition of MFD driver.
The existing touchsreen driver is a MFD client.
For better readability we rename the file to
indicate its functionality as only touchscreen.

Signed-off-by: Patil, Rachna <rachna@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>