cxl/registers: Fix Documentation warning

Commit 0f06157 ("cxl/core: Move register mapping infrastructure")
neglected to add a DOC header for the new drivers/core/regs.c file.

Reported-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163072206675.2250120.3527179192933919995.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Author: djbw

Documentation/driver-api/cxl/memory-devices.rst

@@ -43,7 +43,7 @@ CXL Core
    :doc: cxl pmem
 
 .. kernel-doc:: drivers/cxl/core/regs.c
-   :internal:
+   :doc: cxl registers
 
 External Interfaces
 ===================

drivers/cxl/core/regs.c

@@ -1,12 +1,25 @@
 // SPDX-License-Identifier: GPL-2.0-only
 /* Copyright(c) 2020 Intel Corporation. */
-
 #include <linux/io-64-nonatomic-lo-hi.h>
 #include <linux/device.h>
 #include <linux/slab.h>
 #include <linux/pci.h>
 #include <cxlmem.h>
 
+/**
+ * DOC: cxl registers
+ *
+ * CXL device capabilities are enumerated by PCI DVSEC (Designated
+ * Vendor-specific) and / or descriptors provided by platform firmware.
+ * They can be defined as a set like the device and component registers
+ * mandated by CXL Section 8.1.12.2 Memory Device PCIe Capabilities and
+ * Extended Capabilities, or they can be individual capabilities
+ * appended to bridged and endpoint devices.
+ *
+ * Provide common infrastructure for enumerating and mapping these
+ * discrete capabilities.
+ */
+
 /**
  * cxl_probe_component_regs() - Detect CXL Component register blocks
  * @dev: Host device of the @base mapping