ppc64: limited 32-bit DMA address range

Further investigation of issues with 32-bit DMA on PowerNV revealed that
its window is hardcoded by OPAL (at least in skiboot version 5.4.9) and
cannot be changed by the OS.
Thus, now jhb suggestion of limiting the range in PCI DMA tag seems
the best way to deal with it.

Reviewed by: jhibbits, nwhitehorn, sbruno
Approved by: jhibbits(mentor)
Differential Revision: https://reviews.freebsd.org/D17601