ACPI / battery: mark DMI table as __initconst

The bat_dmi_table[] DMI table is referenced from the __init function
acpi_battery_init_async() only. It and its referenced functions can
therefore be marked __initconst to free up ~1kB of runtime memory after
initialization is done.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>