drivers/perf: xgene_pmu: Fix IOB SLOW PMU parser error

This patch fixes the below parser error of the IOB SLOW PMU.

        # perf stat -a -e iob-slow0/cycle-count/ sleep 1
        evenf syntax error: 'iob-slow0/cycle-count/'
                                 \___ parser error

It replaces the "-" character by "_" character inside the PMU name.

Signed-off-by: Hoan Tran <hoan.tran@amperecomputing.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>