asm-generic: make pci.h usable directly

Some generic code is using the horribly misnamed PCI_DMA_BUS_IS_PHYS
from asm/pci.h. This makes sure that an architecture without PCI
support does not have to define this itself but can rely on the
asm-generic version.

Signed-off-by: Remis Lima Baima <remis.developer@googlemail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>