arm64: dts: imx8mm-kontron: Fix polarity of reg_rst_eth2

The regulator reg_rst_eth2 should keep the reset signal of the USB ethernet
adapter deasserted anytime. Fix the polarity and mark it as always-on.

Anyway, using the regulator is only a workaround for the missing support of
specifying a reset GPIO for USB devices in a generic way. As we don't
have a solution for this at the moment, at least fix the current
workaround.

Fixes: 8668d8b2

 ("arm64: dts: Add the Kontron i.MX8M Mini SoMs and baseboards")
Cc: stable@vger.kernel.org
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>