lib: utils: Rename the prefix in PMU DT properties

As per the DT schema rules, the prefix should be vendor. As the PMU
properties are generic for all vendors, change the prefix to riscv
instead of pmu.

Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>