ARC: [plat-hsdk] use actual clk driver to manage cpu clk

With corresponding clk driver now merged upstream, switch to it.

 - core_clk now represent the PLL (vs. fixed clk before)
 - input_clk represent the clk signal src for PLL (basically xtal)

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