hw/rtc: ARM/arm64: Use MMIO at higher addresses

Using the RTC device at its legacy I/O address as set by IBM in 1981
was a kludge we used for simplicity on ARM platforms as well.
However this imposes problems due to their missing alignment and overlap
with the PCI I/O address space.

Now that we can switch a device easily between using ioports and
MMIO, let's move the RTC out of the first 4K of memory on ARM platforms.

That should be transparent for well behaved guests, since the change is
naturally reflected in the device tree.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>