m68k: split heartbeat out of timer function

The heartbeat functionality is mostly separate from the
actual timer interrupt handling, and it is only used on
five platforms.

Split it out into a separate function and call that directly
from the timer irq on those platforms.

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>