net: can: mscan: remove non-CCF code for MPC512x

transition to the common clock framework has completed and the PPC_CLOCK
is no longer available for the MPC512x platform, remove the now obsolete
code path of the mpc5xxx mscan driver which accessed clock control module
registers directly

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>