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/powerpc/mm/icswx.c

@@ -186,7 +186,7 @@ static u32 acop_get_inst(struct pt_regs *regs)
 }
 
 /**
- * @regs: regsiters at time of interrupt
+ * @regs: registers at time of interrupt
  * @address: storage address
  * @error_code: Fault code, usually the DSISR or ESR depending on
  *		processor type