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/arch/arc/kernel/unwind.c b/arch/arc/kernel/unwind.c
index b6e4f7a..333daab 100644
--- a/arch/arc/kernel/unwind.c
+++ b/arch/arc/kernel/unwind.c
@@ -845,7 +845,7 @@
 				    * state->dataAlign;
 				break;
 			case DW_CFA_def_cfa_register:
-				unw_debug("cfa_def_cfa_regsiter: ");
+				unw_debug("cfa_def_cfa_register: ");
 				state->cfa.reg = get_uleb128(&ptr.p8, end);
 				break;
 				/*todo case DW_CFA_def_cfa_expression: */