m68k: Use asm-generic version of <asm/types.h>

The extra definition for BITS_PER_LONG we had is also indirectly provided
by <asm-generic/types.h>, via <asm-generic/int-ll64.h> and
<asm/bitsperlong.h>

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>