driver/perf: Add identifier sysfs file for Yitian 710 DDR

To allow userspace to identify the specific implementation of the device,
add an "identifier" sysfs file.

The perf tool can match the Yitian 710 DDR metric through the identifier.

Signed-off-by: Jing Zhang <renyu.zj@linux.alibaba.com>
Acked-by: Ian Rogers <irogers@google.com>
Reviewed-by: Shuai Xue <xueshuai@linux.alibaba.com>
Link: https://lore.kernel.org/r/1687245156-61215-2-git-send-email-renyu.zj@linux.alibaba.com
Signed-off-by: Will Deacon <will@kernel.org>