ARM64: use GENERIC_PCI_IOMAP

Patch 09a57239

 ("arm64: Use include/asm-generic/io.h") correctly
removed the GENERIC_IOMAP selection from ARM64, which is not needed
on architectures that have memory-mapped PCI I/O space, however
we now lack a pci_iomap() function.

Fortunately, there is already a generic implementation for this
case, so we just need to select GENERIC_PCI_IOMAP to make it all
work.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 09a57239 ("arm64: Use include/asm-generic/io.h")