m68k: set arch_gettimeoffset directly

remove m68k's mach_gettimeoffset function pointer, and instead directly
set the arch_gettimeoffset function pointer. This requires multiplying
all function results by 1000, since the removed m68k_gettimeoffset() did
this. Also, s/unsigned long/u32/ just to make the function prototypes
exactly match that of arch_gettimeoffset.

Cc: Joshua Thompson <funaho@jurai.org>
Cc: Sam Creasey <sammy@sammy.net>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Phil Blundell <philb@gnu.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>