dt: bindings: lm3692x: Add ti,ovp-microvolt property

This allows to set the overvoltage protection to 17V, 21V, 25V or 29V.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Rob Herring <robh@kernel.org>