arm64: dts: imx8mm: Pass a unit name for the 'soc' node

arm64: dts: imx8mm: Pass a unit name for the 'soc' node

The 'soc' name needs a unit name to match its 'ranges' property.

Pass the unit name in order to fix the following dtc build warning
with W=1:

arch/arm64/boot/dts/freescale/imx8mm.dtsi:203.6-754.4: Warning (unit_address_vs_reg): /soc: node has a reg or ranges property, but no unit name

This also aligns with imx8mq.dtsi.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>