scripts/spelling.txt: add regsiter -> register spelling mistake

This typo is quite common.  Fix it and add it to the spelling file so
that checkpatch catches it earlier.

Link: http://lkml.kernel.org/r/20170317011131.6881-2-sboyd@codeaurora.org
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 6d2a63e..473f088 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -72,7 +72,7 @@ ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops);
  * CONTROL, SAVE_REGS, SAVE_REGS_IF_SUPPORTED, RECURSION_SAFE, STUB and
  * IPMODIFY are a kind of attribute flags which can be set only before
  * registering the ftrace_ops, and can not be modified while registered.
- * Changing those attribute flags after regsitering ftrace_ops will
+ * Changing those attribute flags after registering ftrace_ops will
  * cause unexpected results.
  *
  * ENABLED - set/unset when ftrace_ops is registered/unregistered