ST Voltage regulator for FlashSS vsense

This is the voltage regulator driver to manage the vsense inside the ST flash
sub-system that is used for configuring MMC, NAND, SPI voltages.

Required properties:
- compatible: can be "st,vmmc", "st,vnand" or "st,vspi" because it is shared
  among these devices inside the ST FlashSS. Currently it is only tested on
  MMC.

Required properties:
- regulator-name: regulator name.
- reg: is the vsense top config base address in the flashSS hardware.

Any property defined as part of the core regulator
binding, defined in regulator.txt, can also be used.

Example:

vmmc_reg: voltage-regulator {
	compatible = "st,vmmc";
	regulator-name = "vmmc0";
	reg = <0x9061004 0x4>;
};
