ARM: dts: imx6qdl-kontron-samx6i: move phy reset into phy-node

Add ethernet-phy node so we can drop the deprecated fec phy-reset-gpios
property. The reset-assert-us value is taken from the existing logic
since the fec driver will add an 1ms assert delay per default if
phy-reset-gpios is used and phy-reset-duration is not specified.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>