arm64: dts: ti: introduce minimal dt for am625

To aid in reducing the time needed to bring up Linux on a custom device
based around Texas Instrument's am625 SoCs, define the absolute minimum
necessary to start the SoC.

Delete and disable all blocks and peripherals (other than the needed
UARTs to load the initial binaries) not needed to run Linux.

CC: Judith Mendez <jm@ti.com>
Signed-off-by: Bryan Brattlof <bb@ti.com>