ARC: [plat-axs10x]: remove hardcoded video mode from bootargs

Now have pixel clock PLL driver and we can change pixel clock rate
so we don't need to enforce one exact video mode. Moreover enforcing
video mode is harmful in case of we enforce mode which isn't
supported by the monitor we are using.

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