ARM: dt: Add PMU node, making PMU child devices childs of this node

Add the PMU node, and move the child devices of the PMU node beneath
this new node, giving it a "simple-bus" so that the OF platform
device creator will create these child devices.  No functional change
from this is expected.

The PMU provides multiple features, including an interrupt, reset,
power and isolation controller.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>