tools/firmware: add ACPI device for Windows laptop/slate mode switch

tools/firmware: add ACPI device for Windows laptop/slate mode switch

Microsoft have defined an ACPI device to support switching Windows 10
between laptop/desktop mode and slate/tablet mode [1].

This patch adds an SSDT containing such a device. The presence of the
device is controlled by a new 'acpi_laptop_slate' boolean in xl.cfg.
The new device will not be present by default.

[1] https://msdn.microsoft.com/en-us/windows/hardware/commercialize/design/device-experiences/continuum

Signed-off-by: Owen Smith <owen.smith@citrix.com>
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>