regulator: bd9571mwv: Document "backup_mode" sysfs file

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/ABI/testing/sysfs-driver-bd9571mwv-regulator

+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.