next take at setting up a dma mask by default for platform devices

[Christoph Hellwig]

Hi all,

this is another attempt to make sure the dma_mask pointer is always
initialized for platform devices.  Not doing so lead to lots of
boilerplate code, and makes platform devices different from all our
major busses like PCI where we always set up a dma_mask.  In the long
run this should also help to eventually make dma_mask a scalar value
instead of a pointer and remove even more cruft.

The bigger blocker for this last time was the fact that the usb
subsystem uses the presence or lack of a dma_mask to check if the core
should do dma mapping for the driver, which is highly unusual.  So we
fix this first.  Note that this has some overlap with the pending
desire to use the proper dma_mmap_coherent helper for mapping usb
buffers.  The first two patches from this series should probably
go into 5.3 and then uses as the basis for the decision to use
dma_mmap_coherent.