ARM: dts: bcm283x: Add generic USB PHY

In order to use dwc2 in OTG or gadget mode the USB PHY should be
specified. Since there is no bcm283x USB PHY driver use the generic
one.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Eric Anholt <eric@anholt.net>