arm64: dts: imx8mm-kontron: Fix reset delays for ethernet PHY

According to the datasheet the VSC8531 PHY expects a reset pulse of 100 ns
and a delay of 15 ms after the reset has been deasserted. Set the matching
values in the devicetree.

Reported-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>