m68knommu: fix undefined reference to `mach_get_rtc_pll'

Configuring for a nommu classic m68k target and enabling the generic rtc
driver (CONFIG_RTC_DRV_GENERIC) will result in the following compile
error:

   m68k-linux-ld: arch/m68k/kernel/time.o: in function `rtc_ioctl':
   time.c:(.text+0x82): undefined reference to `mach_get_rtc_pll'
   m68k-linux-ld: time.c:(.text+0xbc): undefined reference to `mach_set_rtc_pll'
   m68k-linux-ld: time.c:(.text+0xf4): undefined reference to `mach_set_rtc_pll'

There are no definitions of "mach_set_rtc_pll" and "mach_get_rtc_pll" in the
nommu code paths. Move these definitions and the associated "mach_hwclk",
so that they are around their use case in time.c. This means they will
always be defined on the builds that require them, and not on those that
cannot use them - such as ColdFire (both with and without MMU enabled).

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>