arm64: dts: verdin-imx8mm: improve pcie node

Slight improvement of pcie node to be more in-line with what we add on
the Verdin iMX8M Plus.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>