We often encounter the situation where slow devices should not be
probed during startup since probing is slow and maybe unnecessary
for unused devices. With MMC we have the 'probe' device parameter,
for ata we have the same, for USB we have the 'usb' command. Overall
this is not very consistent.
With MMC there is the additional problem that the probe parameter
is attached to the logical device when we often have the information
which physical device we want to probe.
This patch adds a 'detect' callback for devices and adds a command
to detect devices and to list the devices which are actually detecable.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>