cxl/Documentation: Remove 'mixed' from sysfs mode doc

cxl/Documentation: Remove 'mixed' from sysfs mode doc

Commit 188e9529a606 ("cxl: Remove the CXL_DECODER_MIXED mistake")
removed the mixed mode.

Remove it from the sysfs documentation.

Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20250224-remove-mixed-sysfs-v1-1-a329db313dac@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>