powerpc/dma: dma_set_coherent_mask() should not be GPL only

When turning this from inline to an exported function I was a bit
over-eager and made it GPL only. This prevents the use of pretty much
all non-GPL PCI driver which is a bit over the top. Let's bring it
back in line with other architecture.

Fixes: 817820b0

 ("powerpc/iommu: Support "hybrid" iommu/direct DMA ops for coherent_mask < dma_mask")
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>