asm-generic/io.h: implement pci_remap_cfgspace using ioremap_np

Now that we have ioremap_np(), we can make pci_remap_cfgspace() default
to it, falling back to ioremap() on platforms where it is not available.

Remove the arm64 implementation, since that is now redundant. Future
cleanups should be able to do the same for other arches, and eventually
make the generic pci_remap_cfgspace() unconditional.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>