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>

arch/parisc/kernel/entry.S

@@ -1369,7 +1369,7 @@ nadtlb_nullify:
 
 	/* 
 		When there is no translation for the probe address then we
-		must nullify the insn and return zero in the target regsiter.
+		must nullify the insn and return zero in the target register.
 		This will indicate to the calling code that it does not have 
 		write/read privileges to this address.