dt-bindings: Use full path to other schemas

When referencing other schema, it is preferred to use an absolute path
(/schemas/....), which allows also an seamless move of particular schema
out of Linux kernel to dtschema.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20240505084618.135705-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>