arm: dts: k3-j721e-r5: Clean up inclusion hierarchy

Get rid of k3-j721e-r5-*-u-boot.dtsi as it is not
necessary. Change the inclusion hierarchy to be as follows:

	k3-j721e-<board>.dts---
			       -
			        -->k3-j721e-r5-<board>.dts
			       -
k3-j721e-<board>-u-boot.dtsi---

Reason for explicitly mentioning the inclusion of -u-boot.dtsi in code
although it could've been automatically done by U-Boot is to resolve
some of the dependencies that R5 file requires.

Also remove duplicate phandles while making this shift as well as remove
firmware-loader as it serves no purpose without "phandlepart" property.

Signed-off-by: Neha Malcom Francis <n-francis@ti.com>