ARM: dts: am57xx: sbc-am57x: add basic board support

ARM: dts: am57xx: sbc-am57x: add basic board support

SBC-AM57x is a single board computer designed for industrial and
embedded applications. It is based on the Texas Instruments Sitara AM57x
system-on-chip family. SBC-AM57x is implemented with the CL-SOM-AM57x
computer-on-module providing most of the functions, and SB-SOM-AM57x
carrier board providing additional peripheral functions and connectors.

https://www.compulab.co.il/products/sbcs/sbc-am57x-ti-am5728-am5718-single-board-computer/

https://www.compulab.co.il/products/computer-on-modules/cl-som-am57x-ti-am5728-am5718-system-on-module/

Add basic board support, including UART3, used as a serial console.

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>