iflib: introduce isc_dma_width

Some DMA controllers are unable to address the full host memory space
and are instead limited to a subset of address range (e.g. 48-bit).

Allow the driver to specify the maximum allowed DMA addressing width
(in bits) for the NIC hardware, by introducing a new field in
if_softc_ctx.

If said field is omitted (set to 0), the lowaddr of DMA window bounds
defaults to BUS_SPACE_MAXADDR.

Submitted by: Artur Rojek <ar@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D28706