dt-bindings: nfc: st,st21nfca: convert to dtschema

Convert the ST ST21NFCA NFC controller to DT schema format.

Changes during bindings conversion:
1. Add a new required "interrupts" property, because it was missing in
   the old bindings by mistake.
2. Drop the "clock-frequency" property during conversion because it is a
   property of I2C bus controller, not I2C slave device.  It was also
   never used by the driver.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20211011073934.34340-5-krzysztof.kozlowski@canonical.com


Signed-off-by: Rob Herring <robh@kernel.org>