Make sure to deregister all child devices also on probe errors to avoid
leaks and to fix probe deferral:

cpsw 4a100000.ethernet: omap_device: omap_device_enable() called from invalid state 1
cpsw 4a100000.ethernet: use pm_runtime_put_sync_suspend() in driver?
cpsw: probe of 4a100000.ethernet failed with error -22

Add generic helper to undo the effects of cpsw_probe_dt(), which will
also be used in a follow-on patch to fix further leaks that have been
introduced more recently.

Note that the platform device is now runtime-resumed before registering
any child devices in order to make sure that it is synchronously
suspended after having deregistered the children in the error path.

Fixes: 1fb19aa7 ("net: cpsw: Add parent<->child relation support
between cpsw and mdio")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>