MAINTAINERS: Add bits.h to the BITMAP API record

>From time to time changes are tending to go to the bits.h
headers while it may affect other bit operataions. Add
the bits.h to the BITMAP API record.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Yury Norov <yury.norov@gmail.com>
Acked-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Yury Norov <yury.norov@gmail.com>