arm: dts: imx8mn-u-boot: Create common imx8mn-u-boot.dtsi

Multiple boards create duplicate entries in their respective
-u-boot.dtsi files which all basically do the same thing.
To consolidate these and make it easier to make improvements
going forward, consolidate them all into one place.

This file creates a flash.bin image using binman, and supports
LPDDR4, DDR4 and DDR3.  Since individual boards use different
peripherals and different UART ports, those entries were kept
in their respective board files, but the spba1 node was addded
which contains all UART1-3 to help facilitate SPL_DM_SERIAL.
Individual users will still need to include their respective
UART and pinctrl nodes for those UARTS.

This consolidated file also supports generating a flash.bin file
which can boot from flexSPI if CONFIG_FSPI_CONF_HEADER is
enabled.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>