iio: adc: ad4691: add oversampling support

iio: adc: ad4691: add oversampling support

Add oversampling ratio (OSR) support for CNV burst mode. The accumulator
depth register (ACC_DEPTH_IN(0)) is programmed with the selected OSR at
buffer enable time and before each single-shot read.

Supported OSR values: 1, 2, 4, 8, 16, 32.

Introduce AD4691_MANUAL_CHANNEL() for manual mode channels, which do
not expose the oversampling_ratio attribute since OSR is not applicable
in that mode. A separate manual_channels array is added to
struct ad4691_channel_info and selected at probe time.

The OSR is shared across all channels (in_voltage_sampling_frequency
and in_voltage_oversampling_ratio are info_mask_shared_by_all) because
the chip has one internal oscillator and a single accumulator depth
register (ACC_DEPTH_IN(0)) for all channels.

in_voltage_sampling_frequency represents the effective output rate,
defined as osc_freq / osr. Writing it computes needed_osc = freq * osr
and snaps down to the largest oscillator table entry that satisfies both
osc <= needed_osc and osc % osr == 0, guaranteeing an exact integer
read-back. The result is stored in target_osc_freq_Hz and written to
OSC_FREQ_REG at buffer enable and single-shot time, so sampling_frequency
and oversampling_ratio can be set in any order.

in_voltage_sampling_frequency_available is precomputed at probe for
each OSR value, listing only oscillator table entries that divide
evenly by that OSR, expressed as effective rates (osc_freq / osr).
The list becomes sparser as OSR increases, capping at max_rate / osr.
read_avail picks the precomputed list for the current OSR, making the
returned pointer stable and race-free.

Writing oversampling_ratio stores the new shared OSR and snaps
target_osc_freq_Hz to the largest oscillator table entry that is both
<= old_effective_rate * new_osr and evenly divisible by new_osr. This
preserves an integer read-back of in_voltage_sampling_frequency after
the OSR change while keeping the oscillator as close as possible to the
previous effective rate.

OSR defaults to 1 (no accumulation).

Signed-off-by: Radu Sabau <radu.sabau@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>