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IO domain voltages on some Rockchip SoCs are variable but need to be
kept in sync between the regulators and the SoC using a special
register.

A specific example using rk3288:
- If the regulator hooked up to a pin like SDMMC0_VDD is 3.3V then
  bit 7 of GRF_IO_VSEL needs to be 0.  If the regulator hooked up to
  that same pin is 1.8V then bit 7 of GRF_IO_VSEL needs to be 1.

Said another way, this driver simply handles keeping bits in the SoC's
general register file (GRF) in sync with the actual value of a voltage
hooked up to the pins.

Note that this driver specifically doesn't include:
- any logic for deciding what voltage we should set regulators to
- any logic for deciding whether regulators (or internal SoC blocks)
  should have power or not have power

If there were some other software that had the smarts of making
decisions about regulators, it would work in conjunction with this
driver.  When that other software adjusted a regulator'...