Documentation/driver-api/cxl: BIOS/EFI expectation update

Documentation/driver-api/cxl: BIOS/EFI expectation update

Add a snippet about what Linux expects BIOS/EFI to do (and not
to do) to the BIOS/EFI section.

Suggested-by: Alejandro Lucero Palau <alucerop@amd.com>
Signed-off-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Alejandro Lucero Palau <alucerop@amd.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://patch.msgid.link/20251219170538.1675743-2-gourry@gourry.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>