dt-bindings: qcom: geni-se: document I2C Master Hub wrapper variant

The I2C Master Hub is a stripped down version of the GENI Serial Engine
QUP Wrapper Controller but only supporting I2C serial engines without
DMA support.

Document the variant compatible, forbid UART and SPI sub-nodes,
and remove requirement for the Master AHB clock and iommu property.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>