take the bus_dma_limit into account on arm

From: Christoph Hellwig
Date: Tue Feb 18 2020 - 13:41:41 EST

Hi Russell,

this series fixes the arm dma coherent allocator to take the bus dma
mask into account, similar to what other architectures do. Without
this devices that support 64-bit mask, but are limited by the
interconnect won't work properly.