iio: adc: ad7768-1: Ensure SYNC_IN pulse minimum timing requirement

iio: adc: ad7768-1: Ensure SYNC_IN pulse minimum timing requirement

The SYNC_IN pulse width must be at least 1.5 x Tmclk, corresponding to
~2.5 µs at the lowest supported MCLK frequency. Add a 3 µs delay to
ensure reliable synchronization timing even for the worst-case scenario.

Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/d3ee92a533cd1207cf5c5cc4d7bdbb5c6c267f68.1749063024.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>