ARM: dts: bcm283x: Move arm-pmu out of soc node

The ARM PMU doesn't have a reg address, so fix the following DTC warning
(requires W=1):
Node /soc/arm-pmu missing or empty reg/ranges property

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>