powerpc: mpc512x: array decl for MCLK registers in CCM

reword the clock control module's registers declaration such that the
MCLK related registers form an array and get indexed by PSC controller
or CAN controller component number

this change is in preparation to COMMON_CLK support for the MPC512x
platform, the changed declaration remains neutral to existing code since
the PSC and MSCAN CCR fields declared here aren't referenced elsewhere

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>