dt-bindings: regulator: twl6030: Add retain-on-reset property

During reset the VMMC regulator doesn't reach 0V and only drops to
1.8V, furthermore the pulse width is under 200us whereas the SD
specification expect 1ms.

For this 2 reasons being able to no reset at all the VMMC during warm
reset and keep the current voltage is a good workaround. The TWL6030
allows this but needs to be aware of it and this configuration should
also be shared with the bootloader.

This is the purpose of this new property: ti,retain-on-reset
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Link: https://lore.kernel.org/r/20190725094542.16547-2-gregory.clement@bootlin.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/twl-regulator.txt

@@ -71,3 +71,10 @@ Example:
 		regulator-min-microvolt  = <1000000>;
 		regulator-max-microvolt  = <3000000>;
 	};
+
+For twl6030 regulators/LDOs:
+
+ - ti,retain-on-reset: Does not turn off the supplies during warm
+                       reset. Could be needed for VMMC, as TWL6030
+                       reset sequence for this signal does not comply
+                       with the SD specification.