ACPI: battery: Install Notify() handler directly

Modify the ACPI battery driver to install its own Notify() handler
directly instead of providing an ACPI driver .notify() callback.

This will allow the ACPI driver .notify() callback to be eliminated and
it will allow the battery driver to be switched over to a platform one
in the future.

While at it, fix up whitespaces in acpi_battery_remove().

Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
[ rjw: Subject and changelog edits, whitespace adjustments ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>