dt-bindings: nfc: ti,trf7970a: Restrict the ti,rx-gain-reduction-db values

Instead of stating the supported values for the ti,rx-gain-reduction-db
property in free text format, add an enum entry that can help validating
the devicetree files.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20250826141736.712827-1-festevam@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>