dt-bindings: soc: samsung: usi: refer to dtschema for SPI

After adding DT schema for Samsung SPI controller, the Samsung USI
bindings can reference it directly for proper schema validation.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220324134106.103588-1-krzk@kernel.org