dt-bindings: arm: sparx5: Add documentation for Microchip Sparx5 SoC

This adds the main Sparx5 SoC DT documentation file, with information
abut the supported board types.

Link: https://lore.kernel.org/r/20200615133242.24911-2-lars.povlsen@microchip.com


Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>