m68k: Remove BKL from rtc implementations

m68k does not support SMP. The access to the rtc is already serialized
with local_irq_save/restore which is sufficient on UP.

The open() protection in arch/m68k/mvme16x/rtc.c is not pretty but
sufficient on UP and safe w/o the BKL.

open() in arch/m68k/bvme6000/rtc.c can do with the same atomic logic
as arch/m68k/mvme16x/rtc.c

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>