powerpc/85xx: fix broken cpu "clock-frequency" property

When indexing freqProcessor[] we use the first
value in the cpu's "reg" property, which on
new e6500 cores IDs the threads.
But freqProcessor[] should be indexed with a
core index so, when fixing "the clock-frequency"
cpu node property, access the freqProcessor[]
with the core index derived from the "reg' property.
If we don't do this, last half of the "cpu" nodes
will have broken "clock-frequency" values.

Signed-off-by: Laurentiu Tudor <Laurentiu.Tudor@freescale.com>
Cc: York Sun <yorksun@freescale.com>