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>

Documentation/admin-guide/laptops/thinkpad-acpi.rst

@@ -50,6 +50,7 @@ detailed description):
 	- WAN enable and disable
 	- UWB enable and disable
 	- LCD Shadow (PrivacyGuard) enable and disable
+	- Lap mode sensor
 
 A compatibility table by model and feature is maintained on the web
 site, http://ibm-acpi.sf.net/. I appreciate any success or failure
@@ -1432,6 +1433,20 @@ The first command ensures the best viewing angle and the latter one turns
 on the feature, restricting the viewing angles.
 
 
+DYTC Lapmode sensor
+------------------
+
+sysfs: dytc_lapmode
+
+Newer thinkpads and mobile workstations have the ability to determine if
+the device is in deskmode or lapmode. This feature is used by user space
+to decide if WWAN transmission can be increased to maximum power and is
+also useful for understanding the different thermal modes available as
+they differ between desk and lap mode.
+
+The property is read-only. If the platform doesn't have support the sysfs
+class is not created.
+
 EXPERIMENTAL: UWB
 -----------------