ARM: dts: OMAP2+: Add PMU nodes

Add PMU nodes for OMAP2, OMAP3 and OMAP4460 devices.

Please note that the node for OMAP4460 has been placed in a separate
header file for OMAP4460, because the node is not compatible with
OMAP4430. The node for OMAP4430 is not included because PMU is not
currently supported on OMAP4430 due to the absence of a cross-trigger
interface driver.

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