ofw/disk: Add parsedev support

Add a parsedev support for OpenFirmware disks. We must look at
characteristics of the OFW node to know if we match this device (so
supply a match routine) or not. Add a parsing routine to allocate
devdesc for OpenFirmware disks as well.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37558

(cherry picked from commit 88a8c68298dbc86370d9a7eff06314da398829cb)