m68k: Define rtc_lock on Atari

The nvram and rtc-cmos drivers use the spinlock rtc_lock to protect against
concurrent accesses to the CMOS memory. As m68k doesn't support SMP or preempt
yet, the spinlock calls tend to get optimized away, but not for all
configurations, causing in some rare cases:

| ERROR: "rtc_lock" [drivers/rtc/rtc-cmos.ko] undefined!
| ERROR: "rtc_lock" [drivers/char/nvram.ko] undefined!

Add the spinlock to the Atari core code to avoid this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Michael Schmitz <schmitz@debian.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>