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

Commit 188e952 ("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>

Author: weiny2

Documentation/ABI/testing/sysfs-bus-cxl

@@ -339,14 +339,13 @@ KernelVersion:	v6.0
 Contact:	linux-cxl@vger.kernel.org
 Description:
 		(RW) When a CXL decoder is of devtype "cxl_decoder_endpoint" it
-		translates from a host physical address range, to a device local
-		address range. Device-local address ranges are further split
-		into a 'ram' (volatile memory) range and 'pmem' (persistent
-		memory) range. The 'mode' attribute emits one of 'ram', 'pmem',
-		'mixed', or 'none'. The 'mixed' indication is for error cases
-		when a decoder straddles the volatile/persistent partition
-		boundary, and 'none' indicates the decoder is not actively
-		decoding, or no DPA allocation policy has been set.
+		translates from a host physical address range, to a device
+		local address range. Device-local address ranges are further
+		split into a 'ram' (volatile memory) range and 'pmem'
+		(persistent memory) range. The 'mode' attribute emits one of
+		'ram', 'pmem', or 'none'. The 'none' indicates the decoder is
+		not actively decoding, or no DPA allocation policy has been
+		set.
 
 		'mode' can be written, when the decoder is in the 'disabled'
 		state, with either 'ram' or 'pmem' to set the boundaries for the