Input: touchscreen - move helper functions to core

Input: touchscreen - move helper functions to core

Some devices outside of drivers/input/touchscreen/ can still make
use of the touchscreen helper functions. Therefore, it was agreed
in [1] to move them outside of drivers/input/touchscreen/ so that
other devices can call them without INPUT_TOUCHSCREEN being set.

As part of this change, 'of' is dropped from the filename because
the helpers no longer actually use OF. No changes are made to the
file contents whatsoever.

Based on the feedback in [2], the corresponding binding documents
(touchscreen.yaml and touchscreen.txt) are left in their original
locations.

[1] https://patchwork.kernel.org/patch/11924029/
[2] https://patchwork.kernel.org/patch/12042037/

Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Link: https://lore.kernel.org/r/20210301234928.4298-2-jeff@labundy.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>