pdbg: Re-add probe command
pdbg: Re-add probe command

The update to targeting with device-tree broke probe function which shows
detected/available targets. This was a useful function so add it back in.

Signed-off-by: Alistair Popple <e9adbcf81c8a36ccc628e09bc92d0e25e1c8e501@popple.id.au>

open-power/pdbg,open-power/pdbg,open-power/pdbg