MIPS: mscc: remove unused CPU_CLOCK_RATE

CPU_CLOCK_RATE is just used once for CONFIG_SYS_MIPS_TIMER_FREQ
which is migrated to Kconfig in the next patch.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>