mecp5123: fix build error

The mecp5123 board did not compile because the MSCAN Clock Control
Registers were missing; these got added, but as an array instead
of 4 individual registers. Adapt the code so it builds.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Reinhard Arlt <reinhard.arlt@esd-electronics.com>