ARC: get rate from clk driver instead of reading device tree

We were reading clock rate directly from device tree "clock-frequency"
property of corresponding clock node in show_cpuinfo function.

Such approach is correct only in case cpu is always clocked by
"fixed-clock". If we use clock driver that allows rate to be changed
this won't work as rate may change during the time or even
"clock-frequency" property may not be presented at all.

So this commit replaces reading device tree with getting rate from clock
driver. This approach is much more flexible and will work for both fixed
and mutable clocks.

Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>