platform/x86: thinkpad_acpi: lap or desk mode interface

Newer Lenovo Thinkpad platforms have support to identify whether the
system is on-lap or not using an ACPI DYTC event from the firmware.

This patch provides the ability to retrieve the current mode via sysfs
entrypoints and will be used by userspace for thermal mode and WWAN
functionality

Co-developed-by: Nitin Joshi <njoshi1@lenovo.com>
Signed-off-by: Nitin Joshi <njoshi1@lenovo.com>
Reviewed-by: Sugumaran <slacshiminar@lenovo.com>
Reviewed-by: Bastien Nocera <bnocera@redhat.com>
Signed-off-by: Mark Pearson <markpearson@lenovo.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>