m68k: mvme16x: Convert to clocksource API

Add a platform clocksource by adapting the existing arch_gettimeoffset
implementation.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>