RISC-V: Add a perf core library for pmu drivers

Implement a perf core library that can support all the essential perf
features in future. It can also accommodate any type of PMU implementation
in future. Currently, both SBI based perf driver and legacy driver
implemented uses the library. Most of the common perf functionalities
are kept in this core library wile PMU specific driver can implement PMU
specific features. For example, the SBI specific functionality will be
implemented in the SBI specific driver.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>