dt-bindings: arm: fsl: Add back missing i.MX7ULP binding

In the conversion to DT schema, the addition of the i.MX7ULP binding got
dropped. Add it to the binding schema.

Fixes: a1a38e1f ("dt-bindings: arm: Convert FSL board/soc bindings to json-schema")
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>