arm: dts: introduce am62a7 u-boot dtbs

Introduce the base dts files needed for u-boot or to augment the
linux dtbs for use in the u-boot-spl and u-boot binaries

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Bryan Brattlof <bb@ti.com>