ARC: clocksource: DT based probe

ARC: clocksource: DT based probe

- Remove explicit clocksource setup and let it be done by OF framework
  by defining CLOCKSOURCE_OF_DECLARE() for various timers

- This allows multiple clocksources to be potentially registered
  simultaneouly: previously we could only do one - as all of them had
  same arc_counter_setup() routine for registration

- Setup routines also ensure that the underlying timer actually exists.

- Remove some of the panic() calls if underlying timer is NOT detected as
  fallback clocksource might still be available
  1. If GRFC doesn't exist, jiffies clocksource gets registered anyways
  2. if RTC doesn't exist, TIMER1 can take over (as it is always
     present)

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