sbs-battery: add min design voltage to sbs-battery

sbs-battery has a max design voltage but not a min design voltage field.
The SBS spec only has one design voltage:
http://www.sbs-forum.org/specs/sbdat110.pdf



Currently this is being used for max design voltage.  This patch uses it
for min design voltage as well.

Signed-off-by: Simon Que <sque@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Todd Broch <tbroch@chromium.org>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Sebastian Reichel <sre@kernel.org>