To prepare an upcoming patch adding support for another PMIC that has
different startup and shutdown time, use driver_data of
platform_device_id instead of a fixed extended device attribute.

By doing so, we also remove a lot of nested structures that aren't
useful.

With this patch, a new PMIC can be easily supported by just filling
correctly its ax20x_info structure and adding a platform_device_id.

Moreover, since we get rid of extended attributes, rename
axp20x_store_ext_attr to axp20x_store_attr and axp20x_show_ext_attr to
axp20x_show_attr.
Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>