touchpanel: make it possible for machines to override default device path

In cases where the touchpanel input device isn't correctly detected as touchpanel
/dev/input/touchscreen0 isn't created as symlink so we have to explicitly specify
the correct input node (or fix the kernel).

Signed-off-by: Simon Busch <morphis@gravedo.de>
machine: mako: specify correct touchpanel input device node

Signed-off-by: Simon Busch <morphis@gravedo.de>