Input: atmel_mxt_ts - move input device init into separate function

It is useful to initialise the input device later:
- Screen parameters may not be not known yet, for instance if waiting for
  firmware loader to return.
- Device may be in bootloader mode on probe (but could still be recovered by
  firmware download).

In addition, later devices have a different touchscreen object (T100) which
requires handling differently.

This also reduces the complexity of the probe function.

Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>
Acked-by: Yufeng Shen <miletus@chromium.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>