dt: add documentation of "hotpluggable" memory property

Summarize the "hotpluggable" property of dt memory nodes.

Link: http://lkml.kernel.org/r/1479160961-25840-6-git-send-email-arbab@linux.vnet.ibm.com
Signed-off-by: Reza Arbab <arbab@linux.vnet.ibm.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Alistair Popple <apopple@au1.ibm.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Stewart Smith <stewart@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/devicetree/booting-without-of.txt

@@ -974,6 +974,13 @@ compatibility.
       4Gb. Some vendors prefer splitting those ranges into smaller
       segments, but the kernel doesn't care.
 
+  Additional properties:
+
+    - hotpluggable : The presence of this property provides an explicit
+      hint to the operating system that this memory may potentially be
+      removed later. The kernel can take this into consideration when
+      doing nonmovable allocations and when laying out memory zones.
+
   e) The /chosen node
 
   This node is a bit "special". Normally, that's where Open Firmware