powerpc: Add memory_hotplug_max()

Add a function to get the maximum address that can be hotplug added.
This is needed to calculate the size of the tce table needed to cover
all memory in 1:1 mode.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>