dfu: defer parsing of device string to IO backend

Devices are not all identified by a single integer. To support
this, defer the parsing of the device string to the IO backed, so that
it can apply the appropriate rules.

SPI devices are specified as controller:chip_select. SPI/SF support will
be added soon.

MMC devices can also be specified as controller[.hwpart][:partition] in
many commands, although we don't support that syntax in DFU.

Signed-off-by: Stephen Warren <swarren@nvidia.com>