dt-bindings: mfd: brcm,cru: Add USB 2.0 PHY

Northstar's USB 2.0 PHY is part of the CRU MFD.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>