ARC: clockevent: DT based probe

ARC: clockevent: DT based probe

 - timer frequency is derived from DT (no longer rely on top level
   DT "clock-frequency" probed early and exported by asm/clk.h)

 - TIMER0_IRQ need not be exported across arch code, confined to intc as
   it is property of same

 - Any failures in clockevent setup are considered pedantic and system
   panic()'s as there is no generic fallback (unlike clocksource where
   a jiffies based soft clocksource always exists)

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>