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/[email protected]/
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Drew Fustini <[email protected]>
Suggested-by: Gustavo A. R. Silva <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: pdp7

scripts/spelling.txt

@@ -1028,6 +1028,7 @@ oustanding||outstanding
 overaall||overall
 overhread||overhead
 overlaping||overlapping
+overflw||overflow
 overlfow||overflow
 overide||override
 overrided||overridden