arm64: dts: imx8mm-kontron: Disable pullups for onboard UART signals on BL board

These signals are actively driven by the SoC or by the onboard
transceiver. There's no need to enable the internal pull resistors
and due to silicon errata ERR050080 let's disable the internal ones
to prevent any unwanted behavior in case they wear out.

Fixes: 8668d8b2 ("arm64: dts: Add the Kontron i.MX8M Mini SoMs and baseboards")
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>