dt-bindings: ARM: at91: document sama7g5ek board

Document the SAMA7G5 Evaluation Kit based on sama7g5 SoC.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210628120452.74408-1-eugen.hristev@microchip.com