MAINTAINERS: Add bitfield.h to the BITMAP API record

From time to time changes are tending to go to bitfield.h
header while it may affect other bit operataions. Add
bitfiled.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>