ARM: omap3: enable off mode automatically

Enabling off mode was only reachable deeply hidden
in the debugfs. As powersaving is an important feature,
move the option out of its shady place.
The debugfs file can still be used to override the default.

Use the presence of a device compatible to ti,twl4030-idle or
ti,twl4030-idle-osc-off as an indicator that the board is wired correctly
for off mode.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
[tony@atomide.com: updated to fix a checkpatch warning]
Signed-off-by: Tony Lindgren <tony@atomide.com>