lib: sbi_pmu: Rename fw_counter_value

Rename and reuse fw_counter_value array to save both the counter values
for the SBI firmware events and event data for the SBI platform specific
firmware events.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>