ARM: OMAP2+: Prepare for device-tree PMU support

If device-tree is present, then do not create the PMU device from within
the OMAP specific PMU code. This is required to allow device-tree to
create the PMU device from the PMU device-tree node.

PMU is not currently supported for OMAP4430 (due to a dependency on
having a cross-trigger interface driver) and so ensure that this
indicated on boot with or without device-tree.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Benoit Cousson <benoit.cousson@linaro.org>