loader: r316585 did miss sparc/ofw

This update does add the code to pass partition size to vdev_probe() via
simple callback. Tested via tinderbox build, but not yet with actual boot.
The code can be improved still, but to verify the idea to read media
block size and amedia size has to be confirmed first.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D10302