doc: regulator: Add regulator-force-boot-off binding

The actual support was added in commit fec8c900c8b2 ("power: regulator:
Add support for regulator-force-boot-off"), update the docs to include
this.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
diff --git a/doc/device-tree-bindings/regulator/regulator.txt b/doc/device-tree-bindings/regulator/regulator.txt
index 6c9a021..ddb02b7 100644
--- a/doc/device-tree-bindings/regulator/regulator.txt
+++ b/doc/device-tree-bindings/regulator/regulator.txt
@@ -36,6 +36,7 @@
 - regulator-always-on: regulator should never be disabled
 - regulator-boot-on: enabled by bootloader/firmware
 - regulator-ramp-delay: ramp delay for regulator (in uV/us)
+- regulator-force-boot-off: disabled during the boot stage
 - regulator-init-microvolt: a init allowed Voltage value
 - regulator-state-(standby|mem|disk)
   type: object