hw/acpi: Remove legacy reset handling from vmclock

hw/acpi: Remove legacy reset handling from vmclock

The vmclock device only has a reset method in order to plug its memory
region into the system memory. It was originally done this way in order
to defer the memory_region_add_subregion_overlap() from vmclock_realize(),
but that doesn't seem to be necessary (any longer?).

Still, allowing the platform code to do this is cleaner because it lets
the address be specified by the platform, easing the port to Arm and
other platforms in future. And the platform has to be involved anyway
because of the need to include the device in the ACPI tables (or DT).

So drop the reset method and provide a vmclock_mmio_map() function
instead, called from pc_machine_done().

Shift the ACPI table build into #ifdef CONFIG_ACPI_VMCLOCK too while
we're at it, since it looks like that wouldn't have built when vmclock
wasn't enabled.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>