MIPS: enable GENERIC_FIND_FIRST_BIT

MIPS doesn't have architecture-optimized bitsearching functions,
like find_{first,next}_bit() etc.
It's absolutely harmless to enable GENERIC_FIND_FIRST_BIT as this
functionality is not new at all and well-tested. It provides more
optimized code and saves some .text memory (32 R2):

add/remove: 4/1 grow/shrink: 1/53 up/down: 216/-372 (-156)

Users of for_each_set_bit() like hotpath gic_handle_shared_int()
will also benefit from this.

Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>