powerpc: Export cacheable_memzero as its now used in a driver

The Freescale PowerPC specific gianfar driver (gig-e) uses
cacheable_memzero for performance reasons we need to export
the symbol to allow the driver to be built as a module.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>