cpufreq: convert to a single post-init driver (hooks) instance

cpufreq: convert to a single post-init driver (hooks) instance

This reduces the post-init memory footprint, eliminates a pointless
level of indirection at the use sites, and allows for subsequent
alternatives call patching.

Take the opportunity and also add a name to the PowerNow! instance.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>