iio: flow: add Sensirion SLF3S liquid flow sensor driver

Add a driver for the Sensirion SLF3S family of digital
liquid-flow sensors on I2C.  Currently supported variants are
SLF3S-0600F, SLF3S-1300F and SLF3S-4000B; they share the same
register map and differ only in flow-scale factor and calibrated
measurement range.  The variant (and therefore the scale) is
auto-detected from the product-information register at probe time;
a sensor reporting an unknown sub-type falls back to the variant
named in the device tree / I2C table, so a drop-in replacement part
keeps working on a kernel that does not know its sub-type yet.

Each measurement frame returns a 16-bit signed flow value, a
16-bit signed temperature reading and a status word, each
protected by a CRC-8 byte.  The driver exposes the flow rate as
IIO_VOLUMEFLOW and the temperature as IIO_TEMP via the standard
IIO read_raw / read_scale interface.

The volume-flow scale is reported in m^3/s.  As the per-LSB scale
is on the order of 1e-12 m^3/s, it is emitted as a 64-bit
fixed-point value with femto (1e-15) resolution
(IIO_VAL_DECIMAL64_FEMTO) so the small SI value keeps full
precision.  This relies on the IIO_VAL_DECIMAL64_FEMTO format type
added earlier in this series.

The active calibration medium can be switched at runtime between
the factory-calibrated water and isopropyl-alcohol modes via the
in_volumeflow_medium sysfs attribute; the sensor starts in water
mode after probe.

The sensor has no low-power state of its own, so system suspend
stops the measurement and disables the vdd supply; resume powers
the sensor back up, waits out the power-up time and restarts the
measurement with the previously active medium, following the
scd30/scd4x precedent.

This driver also creates the drivers/iio/flow/ subdirectory and
the corresponding Kconfig/Makefile glue.

Signed-off-by: Wadim Mueller <wafgo01@gmail.com>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>

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