dt-bindings: iio: hmc425a: add entry for HMC540S

Added support for HMC540SLP3E broadband 4-bit Silicon IC digital
attenuator with a 15 dB control range and wide frequency coverage
(0.1 to 8 GHz).

Signed-off-by: Ana-Maria Cusco <ana-maria.cusco@analog.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230816110906.144540-2-ana-maria.cusco@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>