Mark any serial ports that aren't registered by the board files as disabled
in the device-tree files.

In practice, none of the now-disabled ports ended up succeeding device
probing because of the missing clock-frequency property. However,
explicitly marking the devices disabled has the advantage of squashing
the dev_warn() the failed probe causes, and documenting that we intend
the port not to be used, rather than accidentally left out the property.
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>