sbi: sbi_pmu: Improve sbi_pmu_init() error handling

This patch makes the following changes:

- As sbi_platform_pmu_init() returns a negative error code on
  failure, let sbi_pmu_init() print out the error code with
  sbi_dprintf().

- In order to distinguish the SBI_EFAIL error returned by
  sbi_pmu_add_*_counter_map(), return SBI_ENOENT to indicate
  that fdt_pmu_setup() failed to locate "riscv,pmu" node, and
  generic_pmu_init() ignores such case.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>