ARC: [plat-axs103] use clk driver #2: Add core pll node to DT to manage cpu clk

Add core pll node (core_clk) to manage cpu frequency.
core_clk represents pll itself.
input_clk represents clock signal source (basically xtal) which
comes to pll input.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>