treewide: correct "differenciate" and "instanciate" typos

Also add these typos to spelling.txt so checkpatch.pl will look for them.

Link: http://lkml.kernel.org/r/88af06b9de34d870cb0afc46cfd24e0458be2575.1529471371.git.fthain@telegraphics.com.au
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt

@@ -49,7 +49,7 @@ compatible (optional) - standard definition
     - may contain the following strings:
         - shared-dma-pool: This indicates a region of memory meant to be
           used as a shared pool of DMA buffers for a set of devices. It can
-          be used by an operating system to instanciate the necessary pool
+          be used by an operating system to instantiate the necessary pool
           management subsystem if necessary.
         - vendor specific string in the form <vendor>,[<device>-]<usage>
 no-map (optional) - empty property