ACPI: CPPC: Add auto select register read/write support

For some AMD shared memory based systems, the autonomous selection bit
needed to be set explicitly. Add autonomous selection register related
APIs to acpi driver, which amd_pstate driver uses later.

Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Wyes Karny <wyes.karny@amd.com>
[ rjw: Fixed up kerneldoc comments, white space adjustment, subject edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>