[CPUFREQ] Fix section mismatch for powernow_cpu_init in powernow-k7.c

Use __cpuinit instead of __init for the cpufreq_driver
init function like it is done in powernow-k8.c.

This is removing the warning generated when compiling with
the CONFIG_DEBUG_SECTION_MISMATCH=y option.

Signed-off-by: Holger Hans Peter Freyther <holger@moiji-mobile.com>
Signed-off-by: Dave Jones <davej@redhat.com>