arm64: dts: meson: g12b: odroid-n2: add the Ethernet PHY reset line

The reset line of the RTL8211F PHY is routed to the GPIOZ_15 pad.
Describe this in the device tree so the PHY framework can bring the PHY
into a known state when initializing it. GPIOZ_15 doesn't support
driving the output HIGH (to take the PHY out of reset, only output LOW
to reset the PHY is supported). The datasheet states it's an "3.3V input
tolerant open drain (OD) output pin". Instead there's a pull-up resistor
on the board to take the PHY out of reset. The GPIO itself will be set
to INPUT mode to take the PHY out of reset and LOW to reset the PHY,
which is achieved with the flags (GPIO_ACTIVE_LOW | GPIO_OPEN_DRAIN).

Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>