powerpc: add min_direct_dma_addr

This patch adds min_direct_dma_addr to struct dev_archdata.

min_direct_dma_addr can be used to define which is the minimum address
suitable for a DMA operation.
dma_capable() is updated to use this information in the SWIOTLB case.

This will be used later to support the Nintendo Wii video game console
which has limitations performing DMA to memory below 0x10000000 (MEM1).