cxl: Add support for _DSM Function for retrieving QTG ID

cxl: Add support for _DSM Function for retrieving QTG ID

CXL spec v3.0 9.17.3 CXL Root Device Specific Methods (_DSM)

Add support to retrieve QTG ID via ACPI _DSM call. The _DSM call requires
an input of an ACPI package with 4 dwords (read latency, write latency,
read bandwidth, write bandwidth). The call returns a package with 1 WORD
that provides the max supported QTG ID and a package that may contain 0 or
more WORDs as the recommended QTG IDs in the recommended order.

Create a cxl_root container for the root cxl_port and provide a callback
->get_qos_class() in order to retrieve the QoS class. For the ACPI case,
the _DSM helper is used to retrieve the QTG ID and returned. A
devm_cxl_add_root() function is added for root port setup and registration
of the cxl_root callback operation(s).

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/170319621294.2212653.1649682083061569256.stgit@djiang5-mobl3
Signed-off-by: Dan Williams <dan.j.williams@intel.com>