power: supply: olpc_battery: Use DT to get battery version

Avoid using the x86 OLPC platform specific call to get the board
version. That wouldn't work on FDT-based ARM MMP2 platform.

Add the XO 1.5 compatible string too. This is actually not completely
necessary as the battery nodes on XO 1.5 claim to be compatible with
"olpc,xo1-battery", but there are, in fact, differencies.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>