sparc64: fix sparse warning in pci.c

Fix following warning:
pci.c:886:5: warning: symbol 'pci64_dma_supported' was not declared. Should it be static?

Add proper prototype in kernel.h and delete local prototype in iommu.c

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>