Parse /soc/dma-ranges to populate the openbsd,dma-constraint property.

Parse /soc/dma-ranges to populate the openbsd,dma-constraint property.
On the MNT Reform, which uses a i.MX8MQ with 4 GB of memory, this makes
sure that we don't allocate DMA buffers above the 32-bit boundary.  As
it turns out the i.MX8MQ's I/O devices are limited to 32-bit addresses.
The i.MX8MP seems to be better in that regard, though at least the USB
controller is still limited.  That's a bit harder to fix in a dynamic
fashion, but we'll take care of that as soon as someone shows up with
an i.MX8MP with that much memory.

ok kettenis@