Eliminate the last dregs of the old global arm_root_dma_tag.

In the old days, device drivers passed NULL for the parent tag when creating
a new tag, and on arm platforms that resulted in a global tag representing
overall platform constraints being substituted in the busdma code. Now all
drivers use bus_get_dma_tag() and if there is a need to represent overall
platform constraints they will be inherited from a tag supplied by nexus or
some bus driver in the hierarchy.

The only arm platforms still relying on the old global-tag scheme were some
xscale boards with special PCI-bus constraints. This change provides those
constraints through a tag supplied by the xscale PCI bus driver, and
eliminates the few remaining references to the old global var.

Reviewed by: cognet