perf: xgene_pmu: use DEVICE_ATTR_RO macro

Use DEVICE_ATTR_RO() helper instead of plain DEVICE_ATTR(),
which makes the code a bit shorter and easier to read.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20210528014940.4184-1-yuehaibing@huawei.com


Signed-off-by: Will Deacon <will@kernel.org>