iio: frequency: adf4377: add adf4378 support

Add separate handling for adf4378 within the driver.

The main difference between adf4377 and adf4378 is that adf4378 has only
one output which is handled by only one gpio.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://patch.msgid.link/20240729095047.25040-3-antoniu.miclaus@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>