ACPI: NHLT: Introduce API for the table

The table is composed of a range of endpoints with each describing
audio formats they support. Most of the operations involve iterating
over elements of the table and filtering them. Simplify the process by
implementing range of getters.

While the acpi_nhlt_endpoint_mic_count() stands out a bit, it is a
critical component for any AudioDSP driver to know how many digital
microphones it is dealing with.

Signed-off-by: Cezary Rojewski <cezary.rojewski@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>