ACPI: utils: Add api to read _SUB from ACPI

Add a wrapper function to read the _SUB string from ACPI.

Signed-off-by: Stefan Binding <sbinding@opensource.cirrus.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20220707151037.3901050-2-sbinding@opensource.cirrus.com


Signed-off-by: Mark Brown <broonie@kernel.org>