pcie-brcmstb: Remove brcm_to_*, add pcie_to_dma40

The DMA framework performs the necessary address conversions for the
common cases based on the dma-ranges DT property. brcm_to_pci and
brcm_to_cpu were probably designed for non-DT use cases, something
which isn't an issue for 2711. This second level of mapping causes
breakage with a non-identity PCIe<->system address mapping, so remove
it completely.

The non-identity mapping also broke the DMA40 support, so add back a
simple translation from PCIe to sys addresses.

arm64 builds also require a wider coherent_dma_mask, otherwise the
bounce buffer allocation is rejected.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>