m68knommu: remove unused CONFIG_GENERIC_CMOS_UPDATE option

The CONFIG_GENERIC_CMOS_UPDATE switch is always enabled for the non-MMU
m68k case. But the underlying code to support it, update_persistent_clock(),
doesn't end up doing anything on the currently supported non-MMU platforms.
No platforms supply the necessary function support for writing back the RTC.

So lets remove this option and support code. This also brings m68knommu
in line with the m68k, which doesn't enabled this switch either.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>