ARM: dts: sb-som: introduce SB-SOM baseboard

CompuLab SB-SOM baseboard is a carrier board for multiple arm-based SoMs.
It currently supports (with minor adjustments to assembly) CM-T43, CM-T54,
and CM-QS600 modules. It is a building block in the SBC-T43 single board
computer, which consists of cm-t43 on top of sb-som-t43.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Benoit Cousson <bcousson@baylibre.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Dmitry Lifshitz <lifshitz@compulab.co.il>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>