ACPI: add support for loading SSDTs via configfs

New tables can be loaded by creating directories under /config/table/
and writing the AML code into the aml table attribute. Various table
attributes will be readable once the table is successfully loaded.

Unloading tables is not supported at the moment, but it can be easily
implemented once ACPI loading functions provide a table handle to be
used for unloading.

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>