ARC: perf: don't bail setup if pct irq missing in device-tree

Current code inadventely bails if hardware supports sampling/overflow
interrupts, but the irq is missing from device tree.

|
| # perf stat -e cycles,instructions,major-faults,minor-faults ../hackbench
| Running with 10 groups 400 process
| Time: 0.921
|
| Performance counter stats for '../hackbench':
|
|   <not supported>      cycles
|   <not supported>      instructions
|                 0      major-faults
|              8679      minor-faults

This need not be as we can still do simple counting based perf stat.
This unborks perf on HSDK-4xD

Cc: <stable@vger.kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>