powerpc: Fix build with CONFIG_MEMORY_HOTPLUG on some configs

For memory hotplug to work, the MMU code needs to provide the functions
create_section_mapping() and remove_section_mapping() to respectively
map and unmap portions of the linear mapping.

At the moment only hash64 provides these, so we provide weak stubs that
just error out. This fixes the build with configurations such as 64-bit
BookE with CONFIG_MEMORY_HOTPLUG enabled.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>