docs: fix references for DMA*.txt files

As we moved those files to core-api, fix references to point
to their newer locations.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/37b2fd159fbc7655dbf33b3eb1215396a25f6344.1592895969.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

arch/parisc/kernel/pci-dma.c

@@ -3,7 +3,7 @@
 ** PARISC 1.1 Dynamic DMA mapping support.
 ** This implementation is for PA-RISC platforms that do not support
 ** I/O TLBs (aka DMA address translation hardware).
-** See Documentation/DMA-API-HOWTO.txt for interface definitions.
+** See Documentation/core-api/dma-api-howto.rst for interface definitions.
 **
 **      (c) Copyright 1999,2000 Hewlett-Packard Company
 **      (c) Copyright 2000 Grant Grundler