Add hotplug support to mcp251x driver

Chip model can now be selected directly by matching the modalias name
(instead of filling the .model field in platform_data), and allows the
module to be auto-loaded. Previous behaviour is of course still supported.

Convert the two in-tree users to this feature (icontrol & zeus).
Tested on an Zeus platform (mcp2515).

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Acked-by: Christian Pellegrin <chripell@fsfe.org>
Cc: Edwin Peer <epeer@tmtservices.co.za>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: David S. Miller <davem@davemloft.net>