power: supply: olpc_battery: Avoid using platform_info

This wouldn't work on the DT-based ARM platform. Let's read the EC version
directly from the EC driver instead.

This removes x86 specific bits that would prevent this driver from being
used with the EC of ARM-based OLPC XO 1.75.

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