memhp: don't generate memory hotplug AML if it's not enabled/supported

That reduces DSDT by 910 bytes when memory hotplug
isn't enabled.

While doing so drop intermediate variables/arguments
passing around ACPI_MEMORY_HOTPLUG_IO_LEN and making
it local to memory_hotplug.c, hardcoding it there as
it can't change.

Also don't pass around ACPI_MEMORY_HOTPLUG_BASE through
intermediate variables/arguments where it's not needed.
Instead initialize in module static variable when MMIO
region is mapped and use that within memory_hotplug.c
whenever it's required.
That way MMIO base specified only at one place and AML
with MMIO would always use the same value.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>