arm: dts: Create common rz-g2-beacon-u-boot file

The rzg2_beacon_defconfig creates an image for RZ/G2[MNH] and
as such creates three different device trees and each of them
have a corresponding -u-boot.dtsi file which are basically
copies of each other.  Create a common include file to be
referenced by each of the respective board-u-boot.dtsi files
to reduce duplicate code and simplify support going forward.
This also restores some lost functionality from the device
tree re-sync and updates the MAINTAINER file to include all
beacon-renesom device tree files.

Signed-off-by: Adam Ford <aford173@gmail.com>