dma-mapping: Add get_required_mask if arch overrides default

If an architecture sets ARCH_HAS_DMA_GET_REQUIRED_MASK and has settable
dma_map_ops, the required mask may change by the ops implementation.
For example, a system that always has an mmu inline may only require 32
bits while a swiotlb would desire bits to cover all of memory.

Therefore add the field if the architecture does not use the generic
definition of dma_get_required_mask. The first use will by by powerpc.
Note that this does add some dependency on the order in which files are
visible here.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>