[POWERPC] Make walk_memory_resource available with MEMORY_HOTPLUG=n

The ehea driver was recently changed[1] to use walk_memory_resource() to
detect the system's memory layout.  However, walk_memory_resource() is
available only when memory hotplug is enabled.  So CONFIG_EHEA was
made to depend on MEMORY_HOTPLUG [2], but it is inappropriate for a
network driver to have such a dependency.

Make the declaration of walk_memory_resource() and its powerpc
implementation (ehea is powerpc-specific) unconditionally available.

[1] 48cfb14f
    "ehea: Add DLPAR memory remove support"

[2] fb7b6ca2


    "ehea: Add dependency to Kconfig"

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Acked-by: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>