The driver looks for "vib-overdrive-mv" property, while in
documentation we have "vib-overdriver-mv". Fix the doc.

Cc: Francesco Diotalevi <francesco.diotalevi@iit.it>
Signed-off-by: NAndrea Merello <andrea.merello@gmail.com>
Signed-off-by: NRob Herring <robh@kernel.org>