cxl/port: Enable HDM Capability after validating DVSEC Ranges

CXL memory expanders that support the CXL 2.0 memory device class code
include an "HDM Decoder Capability" mechanism to supplant the "CXL DVSEC
Range" mechanism originally defined in CXL 1.1. Both mechanisms depend
on a "mem_enable" bit being set in configuration space before either
mechanism activates. When the HDM Decoder Capability is enabled the CXL
DVSEC Range settings are ignored.

Previously, the cxl_mem driver was relying on platform-firmware to set
"mem_enable". That is an invalid assumption as there is no requirement
that platform-firmware sets the bit before the driver sees a device,
especially in hot-plug scenarios. Additionally, ACPI-platforms that
support CXL 2.0 devices also support the ACPI CEDT (CXL Early Discovery
Table). That table outlines the platform permissible address ranges for
CXL operation. So, there is a need for the driver to set "mem_enable",
and there is information available to determine the validity of the CXL
D...