powerpc: remove unncesary swiotlb_arch_address_needs_mapping

swiotlb doesn't use swiotlb_arch_address_needs_mapping(); it uses
dma_capalbe(). We can remove unnecessary
swiotlb_arch_address_needs_mapping().

We can remove swiotlb_addr_needs_map() and is_buffer_dma_capable() in
swiotlb_pci_addr_needs_map() too; dma_capable() handles the features
that both provide.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Becky Bruce <beckyb@kernel.crashing.org>