docs: move xtensa documentation under Documentation/arch/

Architecture-specific documentation is being moved into Documentation/arch/
as a way of cleaning up the top-level documentation directory and making
the docs hierarchy more closely match the source hierarchy.  Move
Documentation/xtensa into arch/ and fix all in-tree references.

Cc: Chris Zankel <chris@zankel.net>
Reviewed-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/arch/index.rst

@@ -25,4 +25,4 @@ implementation.
    ../sh/index
    ../sparc/index
    x86/index
-   ../xtensa/index
+   xtensa/index

arch/xtensa/include/asm/initialize_mmu.h

@@ -43,7 +43,7 @@
 #if XCHAL_HAVE_S32C1I && (XCHAL_HW_MIN_VERSION >= XTENSA_HWVERSION_RC_2009_0)
 /*
  * We Have Atomic Operation Control (ATOMCTL) Register; Initialize it.
- * For details see Documentation/xtensa/atomctl.rst
+ * For details see Documentation/arch/xtensa/atomctl.rst
  */
 #if XCHAL_DCACHE_IS_COHERENT
 	movi	a3, 0x25	/* For SMP/MX -- internal for writeback,