lib: sbi_pmu: Introduce fw_counter_write_value API

Add fw_counter_write_value API for platform specific firmware events
which separates setting the counter's initial value from starting the
counter. This is required so that the fw_event_data array can be reused
to save the event data received.

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