powerpc/pseries: Fix VPHN build errors on non-SMP systems

The header asm/hvcall.h was previously included indirectly via
smp.h. On non-SMP systems, however, these declarations are excluded
and the build breaks. This is easily fixed by including asm/hvcall.h
directly.

The VPHN feature is only meaningful on NUMA systems that implement
the SPLPAR option, so exclude the VPHN code on systems without
SPLPAR enabled.

Also, expose unmap_cpu_from_node() on systems with SPLPAR enabled,
even if CONFIG_HOTPLUG_CPU is disabled.

Lastly, map_cpu_to_node() is now needed by VPHN to manipulate the
node masks after boot time, so remove the __cpuinit annotation to
fix a section mismatch.

Signed-off-by: Jesse Larrew <jlarrew@linux.vnet.ibm.com>