arm/arm64: Fix PCI I/O ports region overlap

The PCI I/O ports region has a size of 64K and starts at address 0.
Unfortunately, this means it overlaps with the UART port addresses (0x2E8,
0x3E8, 0x2F8 and 0x3F8) and the RTC (0x70 and 0x71). Some software might
depend on those fixed addresses for the 8250 UART and RTC (like EDK2 or
kvm-unit-tests), so let's move the PCI I/O ports region instead.

Keep the size of the PCI I/O ports region (64K) and move it at the top of
the PCI MMIO memory, just below system memory. Also make it possible for
any architecture to have their own PCI I/O ports region address.