ARM: dts: Put Cygnus core components under core bus

Put all Cygnus core components into "core" node of type "simple-bus" in
bcm-cygnus.dtsi

Signed-off-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>