[ARM] pxa/MioA701: improve power supply sources

Take advantage of the newly created wm97xx battery driver
and remove useless code in mioa701 board code. Add also the
ac connection detect capability after the matching gpio was
discovered.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Eric Miao <eric.miao@marvell.com>