timekeeping: Simplify arch_gettimeoffset()

Provide a default stub function instead of having the extra
conditional. Cuts binary size on a m68k build by ~100 bytes.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>