MAINTAINERS: Add entry for RISC-V PMU drivers

Add myself and Anup as maintainer for RISC-V PMU drivers.

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>