MAINTAINERS: cpufreq: amd-pstate: Add co-maintainers and reviewer

I'm happy to add Gautham and Mario as the co-maintainers, Perry as the
reviewer for amd-pstate driver.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>