arm64: dts: imx8mn: Create EVK dtsi file for common use

i.MX8MN has different EVK boards to support different DDR types,
the ONLY differences are DDR chips and PMIC, so most of the devices
can be shared between these EVK boards, create a EVK dtsi file for
common use.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>