iio: frequency: adf4371: Add support for output stage mute

Another feature of the ADF4371/ADF4372 is that the supply current to the
RF8P and RF8N output stage can shut down until the ADF4371 achieves lock
as measured by the digital lock detect circuitry. The mute to lock
detect bit (MUTE_LD) in REG25 enables this function.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>