platform/x86: ideapad-laptop: Add sysfs interface for touchpad state

Lenovo Yoga (many variants: Yoga, Yoga2 Pro, Yoga2 13, Yoga3 Pro, Yoga 3
14, etc) has multiple modles that are a hybrid laptop, working in laptop
mode as well as tablet mode.

Currently, there is no easy interface to determine the touchpad status,
which in case of the Yoga family of machines, can also be useful to
assume tablet mode status.

Note: The ideapad-laptop driver does not provide a SW_TABLET_MODE either.

For a detailed discussion  on why we want either of the interfaces,
please see:
https://bugs.launchpad.net/onboard/+bug/1366421/comments/43



This patch adds a sysfs interface for read/write access under:
/sys/bus/platform/devices/VPC2004\:00/touchpad_mode

Signed-off-by: Ritesh Raj Sarraf <rrs@debian.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>