arm: dts: imx8mn-var-som: fix PHY detection bug by adding deassert delay

arm: dts: imx8mn-var-som: fix PHY detection bug by adding deassert delay

While testing the ethernet interface on a Variscite symphony carrier
board using an imx8mn SOM with an onboard ADIN1300 PHY (EC hardware
configuration), the ethernet PHY is not detected.

The ADIN1300 datasheet indicate that the "Management interface
active (t4)" state is reached at most 5ms after the reset signal is
deasserted.

The device tree in Variscite custom git repository uses the following
property:

    phy-reset-post-delay = <20>;

Add a new MDIO property 'reset-deassert-us' of 20ms to have the same
delay inside the ethphy node. Adding this property fixes the problem
with the PHY detection.

Note that this SOM can also have an Atheros AR8033 PHY. In this case,
a 1ms deassert delay is sufficient. Add a comment to that effect.

Fixes: c4c1ed68c1e8 ("imx8mn_var_som: Add support for Variscite
VAR-SOM-MX8M-NANO board")

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>