ARC: [plat-axs103] use clk driver #1: Get rid of platform specific cpu clk setting

historically axs103 platform code used to set the cpu clk by writing to
PLL registers directly. however the axs10x clk driver is now upstream so
no need to do this amymore.

Driver is selected automatically when CONFIG_ARC_PLAT_AXS10X is set

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
[vgupta: deleted more code not needed anymore]