cpufreq: amd-pstate: move to use bus_get_dev_root()

cpufreq: amd-pstate: move to use bus_get_dev_root()

Direct access to the struct bus_type dev_root pointer is going away soon
so replace that with a call to bus_get_dev_root() instead, which is what
it is there for.

In doing so, remove the unneded kobject structure that was only being
created to cause a subdirectory for the attributes.  The name of the
attribute group is the correct way to do this, saving code and
complexity as well as allowing the attributes to properly show up to
userspace tools (the raw kobject would not allow that.)

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: linux-pm@vger.kernel.org
Acked-by: Huang Rui <ray.huang@.amd.com>
Link: https://lore.kernel.org/r/20230313182918.1312597-20-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>