sh: Don't enable GENERIC_TIME for the CMT clockevent driver yet.

GENERIC_TIME still depends on the clocksource bits being there, which is
presently not supported. This allows the CMT clockevent driver to be used
alongside alternate system timers that do not yet provide a clocksource
of their own (MTU2 and so on in the case of SH-2A).

Signed-off-by: Paul Mundt <lethal@linux-sh.org>