lib: sbi_pmu: Align the event type offset as per SBI specification

The bits encoded in event_idx[19:16] indicate the event type, with
an offset of 16 instead of 20.

Fixes: 13d40f21

 ("lib: sbi: Add PMU support")
Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>