arm: dts: imx8mp: add of-list support to common imx8mp-u-boot.dtsi

arm: dts: imx8mp: add of-list support to common imx8mp-u-boot.dtsi

Add support for OF-LIST to common imx8mp-u-boot.dtsi so that it can
be used with boards that have multiple DTB's.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP i.MX U-Boot Team <uboot-imx@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Teresa Remmet <t.remmet@phytec.de>
Cc: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Cc: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>