ACPI: TAD: Add low-level support for real time capability

Add low-level support for the (optional) real time capability of the
ACPI Time and Alarm Device (TAD) to the ACPI TAD driver.

This allows the real time to be acquired or set via sysfs with the
help of the _GRT and _SRT methods of the TAD, respectively.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>