realtek: use fixed-clock as CPU clock

realtek: use fixed-clock as CPU clock

CPU clock definition in devicetree should be specified per CPU, not in
the cpus node.

The implementation of plat_init_time() needs to be modified to look for
the clock frequency in the changed location. To achieve this, the
relevant code is copied from the generic MIPS implementation in
arch/mips/generic/init.c.

Signed-off-by: Sander Vanheule <sander@svanheule.net>