scripts/spelling.txt: add "overflw"

Add typo "overflw" for "overflow".  This typo was found and fixed in
drivers/clocksource/timer-pistachio.c.

Link: https://lore.kernel.org/lkml/20210305090315.384547-1-drew@beagleboard.org/
Link: https://lkml.kernel.org/r/20210305095151.388182-1-drew@beagleboard.org


Signed-off-by: Drew Fustini <drew@beagleboard.org>
Suggested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>