OMAP4: DMTIMER: enable autoidle mode

Currently autoidle is only enabled for OMAP2/3; by enabling autoidle,
the automatic L4 clock gating strategy is applied based on L4 activity,
otherwise L4 clock to module will be a free running.

Signed-off-by: Ambresh K <ambresh@ti.com>
[tony@atomide.com: updated for timer init changes]
Signed-off-by: Tony Lindgren <tony@atomide.com>