cxl: Add KUnit tests for HDM decoder encoding helpers

The CXL subsystem has no KUnit infrastructure for testing self-contained
logic.

Introduce CONFIG_CXL_KUNIT_TEST and drivers/cxl/.kunitconfig, along with
an initial suite for the HDM decoder interleave encoding helpers. The
suite checks valid and invalid encodings and round-trip conversions.

Use a single Kconfig option for all CXL KUnit suites, with each suite
built as a separate module. This allows new suites to be added without
requiring additional configuration options.

The HDM decoder helpers provide a starting point for establishing the
location, naming, and configuration conventions for future CXL KUnit tests.

Suggested-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
Reviewed-by: Li Ming <ming.li@zohomail.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/8a79de4a04a17fa8b92bee617792719dc6bd5a41.1793645996.git.alison.schofield@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

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