So far all the u-boot specific properties for both r5 and a53 are
placed in k3-am654-base-board-u-boot.dtsi. But there are few a53
nodes that should be updated but doesn't belong to r5. So create a
separate r5 specific u-boot dtsi.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210622063431.3151-4-lokeshvutla@ti.com