olpc_battery: Bind to device tree

This is cleaner, and allows suspend/resume (wakeup) handlers to be
added in an upcoming patch.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>