Introduce a regulator_can_change_voltage() function for the subsytems or
drivers which might check if applying voltage change is possible and use
special workaround code when the driver is used with fixed regulators or
regulators with disabled ability to change the voltage.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

Some drivers (at least 3 drivers) have such variant of linear mapping that
the first few selectors are invalid and the reset are linear mapping.
Let's support this case in core.

This patch adds linear_min_sel in struct regulator_desc,
so we can allow specific minimal selector for starting linear mapping.
Then extends regulator_[map|list]_voltage_linear() to support this feature.

Note that for selectors less than min_linear_index, we need count them to
n_voltages so regulator_list_voltage() won't fail while checking the boundary
for selector before calling list_voltage callback.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>