regulator: Updates for v4.18

Quite a lot of core work this time around, though not 100% successful.

We gained support for runtime mode changes thanks to David Collins and
improved support for write only regulators (ones where we can't read
back the configuration) from Douglas Anderson.

There's been quite a bit of work from Linus Walleij on converting from
specfying GPIOs by numbers to descriptors.  Sadly the testing turned out
to be less good than we had hoped and so a lot of this had to be
reverted.

We also have the start of updates to use coupled regulators from Maciej
Purski, unfortunately there are further problems there so the last
couple of patches have been reverted.

We also have new drivers for BD71837 and SY8106A devices, SAW regulators
on Qualcomm SPMI and dropped support for some preproduction chips
that never made it to market from the AB8500 driver.
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Merge branch 'regulator-4.17' into regulator-4.18 merge window