arc: Get rid of root core-frequency property

arc: Get rid of root core-frequency property

Now when we switched to usage of real clk devices for CPU core
frequency those root properties make no sense any longer.
Se we're just getting rid of them here to not confuse readers of
our .dts files.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Christian Ruppert <christian.ruppert@alitech.com>
Cc: Noam Camus <noamca@mellanox.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>