lib: add linear ranges helpers

Many devices have control registers which control some measurable
property. Often a register contains control field so that change in
this field causes linear change in the controlled property. It is not
a rare case that user wants to give 'meaningful' control values and
driver needs to convert them to register field values. Even more
often user wants to 'see' the currently set value - again in
meaningful units - and driver needs to convert the values it reads
from register to these meaningful units. Examples of this include:

- regulators, voltage/current configurations
- power, voltage/current configurations
- clk(?) NCOs

and maybe others I can't think of right now.

Provide a linear_range helper which can do conversion from user value
to register value 'selector'.

The idea here is stolen from regulator framework and patches refactoring
the regulator helpers to use this are following.

Current implementation does not support inversely proportional ranges
but it might be useful if we could support also inversely proportional
ranges?

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/59259bc475e0c800eb4bb163f02528c7c01f7b3a.1588944082.git.matti.vaittinen@fi.rohmeurope.com
Signed-off-by: Mark Brown <broonie@kernel.org>

lib/Kconfig

@@ -19,6 +19,9 @@ config RAID6_PQ_BENCHMARK
 	  Benchmark all available RAID6 PQ functions on init and choose the
 	  fastest one.
 
+config LINEAR_RANGES
+	tristate
+
 config PACKING
 	bool "Generic bitfield packing and unpacking"
 	default n