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>

drivers/thermal/intel_powerclamp.c

@@ -28,7 +28,7 @@
  *              case of external interrupts without need for ack, clamping down
  *              cpu in non-irq context does not reduce irq. for majority of the
  *              cases, clamping down cpu does help reduce irq as well, we should
- *              be able to differenciate the two cases and give a quantitative
+ *              be able to differentiate the two cases and give a quantitative
  *              solution for the irqs that we can control. perhaps based on
  *              get_cpu_iowait_time_us()
  *