ARM: dts: twl4030: Add missing madc reference for bci subnode

The twl4030_charger driver expects an iio channel to detect the
presence of an AC charger by looking at VAC (madc channel 11).
This definition is missing in the device tree.

Signed-off-by: Marek Belisko <marek@goldelico.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>