NFC: trf7970a: Create device-tree parameter for RX gain reduction

The TRF7970a device is sensitive to RF disturbances, which can make it
hard to pass some EMC immunity tests. By reducing the RX antenna gain,
the device becomes less sensitive to EMC disturbances, as a trade-off
against antenna performance.

Add a device tree option to select RX gain reduction to improve EMC
performance.

Selecting a communication standard in the ISO control register resets
the RX antenna gain settings. Therefore set the RX gain reduction
everytime the ISO control register changes, when the option is used.

Signed-off-by: Paul Geurts <paul.geurts@prodrive-technologies.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20250626141242.3749958-3-paul.geurts@prodrive-technologies.com


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