Documentation/admin-guide: mark memmap parameter is supported by a few architectures

early_param memmap is only implemented on X86, MIPS and XTENSA. To avoid
wasting users’ time on trying this on platform like ARM, mark it clearly.
Signed-off-by: NBarry Song <song.bao.hua@hisilicon.com>
Reviewed-by: NMike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/20201128195121.2556-1-song.bao.hua@hisilicon.comSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.rst

@@ -172,6 +172,7 @@ parameter is applicable::
 	X86	Either 32-bit or 64-bit x86 (same as X86-32+X86-64)
 	X86_UV	SGI UV support is enabled.
 	XEN	Xen support is enabled
+	XTENSA	xtensa architecture is enabled.
 
 In addition, the following text indicates that the option::
 

Documentation/admin-guide/kernel-parameters.txt

@@ -2709,7 +2709,7 @@
 			option description.
 
 	memmap=nn[KMG]@ss[KMG]
-			[KNL] Force usage of a specific region of memory.
+			[KNL, X86, MIPS, XTENSA] Force usage of a specific region of memory.
 			Region of memory to be used is from ss to ss+nn.
 			If @ss[KMG] is omitted, it is equivalent to mem=nn[KMG],
 			which limits max address to nn[KMG].