iio: adc: ad7380: prepare for parts with more channels

The current driver supports only parts with 2 channels.
In order to prepare the support of new compatible ADCs with more
channels, this commit:
  - defines MAX_NUM_CHANNEL to specify the maximum number of
    channels currently supported by the driver
  - adds available_scan_mask member in ad7380_chip_info structure
  - fixes spi xfer struct len depending on number of channels
  - fixes scan_data.raw buffer size to handle more channels
  - adds a timing specifications structure in ad7380_chip_info structure

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://lore.kernel.org/r/20240528-adding-new-ad738x-driver-v7-5-4cd70a4c12c8@baylibre.com


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