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

Explicitly reference the dtschema for USI children implementing specific
serial protocol (I2C, SPI, UART).  The SPI schema is not yet accepted,
so it will be provided later.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Link: https://lore.kernel.org/r/20220211164716.120880-1-krzysztof.kozlowski@canonical.com