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