lib: utils/fdt: Fix DT parsing in fdt_pmu_setup()

This patch does following fixes in fdt_pmu_setup():
1) If any of the event mapping DT property is absent or too small
   then don't skip parsing of other DT properties.
2) Return failure if sbi_pmu_add_hw_event_counter_map() fails.
3) Return failure if sbi_pmu_add_raw_event_counter_map() fails.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>