Many SPI controllers need to add properties to peripheral devices. This
could be the delay in clock or data lines, etc. These properties are
controller specific but need to be defined in the peripheral node
because they are per-peripheral and there can be multiple peripherals
attached to a controller.

If these properties are not added to the peripheral binding, then the
dtbs check emits a warning. But these properties do not make much sense
in the peripheral binding because they are controller-specific and they
will just pollute every peripheral binding. So this binding is added to
collect all such properties from all such controllers. Peripheral
bindings should simply refer to this binding and they should be rid of
the warnings.

There are some limitations with this approach. Firstly, there is no way
to specify required properties. The schema contains properties for all
controllers and there is no way to know which controller is being used.
Secondly, there is no way to restrict additional properties. Since this
schema will be used with an allOf operator, additionalProperties needs
to be true. In addition, the peripheral schema will have to set
unevaluatedProperties: false.

Despite these limitations, this appears to be the best solution to this
problem that doesn't involve modifying existing tools or schema specs.
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211109181911.2251-2-p.yadav@ti.comSigned-off-by: Mark Brown <broonie@kernel.org>