drivers: CCI: fix the error handle in cci_pmu_probe()

This patch fix the error handle of function cci_pmu_probe():
- using IS_ERR() instead of NULL test for the return value of
  devm_ioremap_resource() since it nerver return NULL.
- remove kfree() for devm_kzalloc allocated memory
- remove dev_warn() since devm_ioremap_resource() has error message
  already.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Will Deacon <will.deacon@arm.com>