Document monitored-battery and ti,system-minimum-microvolt properties.

Cc: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Liam Breck <kernel@networkimprov.net>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>