arm64: dts: imx8mn-var-som-symphony: adapt FEC pinctrl for SOMs with onboard PHY

The VAR SOM symphony carrier board can be used with SOMs which have a
soldered ethernet PHY onboard and with SOMs which don't have one.

For SOMs with an onboard PHY, the PHY on the cartrier board is not
used, and GPIO1_IO9 is used as a reset line to the onboard PHY.

For SOMs without an onboard PHY, the PHY on the carrier board is
used. For this configuration, pca9534 GPIO 5 (located on the carrier
board) is used as a reset line to the PHY, and GPIO1_IO9 is not
used.

GPIO1_IO9 is not connected to any user-accessible pins or functions,
and leaving it enabled in the mux pinctrl for both configurations is
safe.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>