arm64: dts: imx8mn-var-som: add missing pull-up for onboard PHY reset pinmux

For SOMs with an onboard PHY, the RESET_N pull-up resistor is
currently deactivated in the pinmux configuration. When the pinmux
code selects the GPIO function for this pin, with a default direction
of input, this prevents the RESET_N pin from being taken to the proper
3.3V level (deasserted), and this results in the PHY being not
detected since it is held in reset.

Taken from RESET_N pin description in ADIN13000 datasheet:
    This pin requires a 1K pull-up resistor to AVDD_3P3.

Activate the pull-up resistor to fix the issue.

Fixes: ade0176d ("arm64: dts: imx8mn-var-som: Add Variscite VAR-SOM-MX8MN System on Module")
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>