Convert the ST NCI (ST21NFCB) NFC controller to DT schema format.

Changes during bindings conversion:
1. Add a new required "reg" property for SPI binding, because SPI child
   devices use it as chip-select.
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/20211011073934.34340-7-krzysztof.kozlowski@canonical.comSigned-off-by: Rob Herring <robh@kernel.org>