dt-bindings: arm: ti: Add binding for AM68 SK

AM68 Starter Kit is a low cost, small form factor board designed for
TI's AM68 SoC which is optimized to provide best in class performance
for industrial applications and add binding for the same.

Signed-off-by: Sinthu Raja <sinthu.raja@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20230116071446.28867-2-sinthu.raja@ti.com