ARM: dts: imx6qdl-nitrogen6x: fix USB PHY reset

Declared as a regulator since the driver doesn't have a reset-gpios
property for this.

This ensures that the PHY is woken up, not depending on the state the
second stage bootloader leaves the pin.

This is a workaround until a proper mechanism is provided to reset such
devices like the pwrseq library [1] for instance.

[1] https://lkml.org/lkml/2017/2/10/779

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>