dts: stm32: Move usb PHY nodes out of SoC to fix warning

We currently get a number of warnings like:

	Warning (simple_bus_reg): /soc/otgfs_phy: missing or empty
	reg/ranges property

This is due to the usb phy nodes not have a reg property since they
don't have an mmio address associated with them.

Move the phy nodes out of the SoC node so their lack of a reg property
will not cause a warning.  This is similar to how Linux dts files
handle the phy nodes.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>