cpufreq_ at32ap-cpufreq.c: Fix section mismatch

The function at32_cpufreq_driver_init was marked as __init but will be
called from inside the cpufreq framework. This lead to the following a
section mismatch during compilation:

WARNING: drivers/built-in.o(.data+0x2448): Section mismatch in reference
from the variable at32_driver to the function
.init.text:at32_cpufreq_driver_init()
The variable at32_driver references
the function __init at32_cpufreq_driver_init()
If the reference is valid then annotate the
variable with __init* or __refdata (see linux/init.h) or name the
variable:
*_template, *_timer, *_sht, *_ops, *_probe, *_probe_one, *_console

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>