net: can: mscan: adjust to common clock support for mpc512x

implement a .get_clock() callback for the MPC512x platform which uses
the common clock infrastructure (eliminating direct access to the clock
control registers from within the CAN network driver), and provide the
corresponding .put_clock() callback to release resources after use

acquire both the clock items for register access ("ipg") as well as for
wire communication ("can")

keep the previous implementation of MPC512x support in place during
migration, this results in a readable diff of the change

this change is neutral to the MPC5200 platform

Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Cc: linux-can@vger.kernel.org
Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>