iio: amplifiers: hmc425a: Add Support HMC540S 4-bit Attenuator

This adds support for the Analog Devices HMC540s 1 dB LSB
Silicon MMIC 4-Bit Digital Positive Control Attenuator, 0.1 - 8 GHz

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Ana-Maria Cusco <ana-maria.cusco@analog.com>
Link: https://lore.kernel.org/r/20230816110906.144540-1-ana-maria.cusco@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>