regulator: bd9571mwv: Document "backup_mode" sysfs file

What:		/sys/bus/i2c/devices/.../bd9571mwv-regulator.*.auto/backup_mode

Date:		Jul 2018

KernelVersion:	4.19

Contact:	Geert Uytterhoeven <geert+renesas@glider.be>

Description:	Read/write the current state of DDR Backup Mode, which controls

		if DDR power rails will be kept powered during system suspend.

		("on"/"1" = enabled, "off"/"0" = disabled).

		Two types of power switches (or control signals) can be used:

		  A. With a momentary power switch (or pulse signal), DDR

		     Backup Mode is enabled by default when available, as the

		     PMIC will be configured only during system suspend.

		  B. With a toggle power switch (or level signal), the

		     following steps must be followed exactly:

		       1. Configure PMIC for backup mode, to change the role of

			  the accessory power switch from a power switch to a

			  wake-up switch,

		       2. Switch accessory power switch off, to prepare for

			  system suspend, which is a manual step not controlled

			  by software,

		       3. Suspend system,

		       4. Switch accessory power switch on, to resume the

			  system.

		     DDR Backup Mode must be explicitly enabled by the user,

		     to invoke step 1.

		See also Documentation/devicetree/bindings/mfd/bd9571mwv.txt.

Users:		User space applications for embedded boards equipped with a

		BD9571MWV PMIC.