pci: separate generic code from x64-specific code

this patch moves the common pci definitions and the prototypes for
the architecture-dependent pci functions from drivers/pci.hh to
arch/common/pci.hh, and puts (most of) the x64-specific PCI stuff
in arch/x64/pci.cc.
Prepare arch-pci.hh with some x64-specific definitions, and build
the new arch/x64/pci.o module.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
aarch64: pci: first implementation

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>