power: supply: core: auto-exposure of simple-battery data

Automatically expose data from the simple-battery firmware
node for all battery drivers.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>