ARM: dts: imx6ul-tx6ul: Switch away from deprecated `phy-reset-gpios`

ARM: dts: imx6ul-tx6ul: Switch away from deprecated `phy-reset-gpios`

The Ethernet PHY's reset GPIO should be specified in the node of the PHY
itself, instead of the MAC (`fec`). The latter is deprecated, and was an
i.MX-specific extension, incompatible with the new reset controller
subsystem.

Co-developed-by: Csaba Buday <buday.csaba@prolan.hu>
Signed-off-by: Csaba Buday <buday.csaba@prolan.hu>
Signed-off-by: Bence Csókás <csokas.bence@prolan.hu>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>