cpupower: Better interface for accessing AMD pci registers

AMD's BKDG (Bios and Kernel Developers Guide) talks in the CPU spec of their
CPU families about PCI registers defined by "device" (slot) and func(tion).

Assuming that CPU specific configuration PCI devices are always on domain
and bus zero a pci_slot_func_init() func which gets the slot and func of
the desired PCI device passed looks like the most convenient way.

This also obsoletes the PCI device id maintenance.

Signed-off-by: Thomas Renninger <trenn@suse.de>
CC: Andreas Herrmann <herrmann.der.user@googlemail.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>