ACPI: memory hotplug: remove .start() method

This patch folds the .start() method into .add().

The .start() method is called in two paths: boot-time device enumeration
and run-time node addition, currently via container_device_add().  In both
cases, .start() is called immediately after .add(), so there's no reason
to make them separate methods.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Reviewed-by: Alex Chiang <achiang@hp.com>
CC: Yasunori Goto <y-goto@jp.fujitsu.com>
CC: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Len Brown <len.brown@intel.com>