arm64: dts: imx8mm: Split the imx8mm evk board dts to a common dtsi

There are two type of i.MX8MM EVK board, one is populated with
LPDDR4(default dts), and one is populated with DDR4. these two
boards share most of the board design, but still have some difference.
imx8mm-evk has emmc support, imx8mm-ddr4-evk has gpmi nand support.
And also, the BT/WIFI module is different. So move the common dts
part into imx8mm-evk.dtsi for reuse.
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>