ARC: Don't try to use value of top level clock-frequency in DT

We no longer use it and instead a real clk device such as fixed-clk
instance is fed to timers etc.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
[vgupta: broken out of a bigger patch, rewrote changelog]
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>