lib: sbi_pmu: Use dedicated event code for platform firmware events

For all platform specific firmware event operations use the dedicated
event code (0xFFFF) when matching against the input firmware event.
Furthermore save the real platform specific firmware event code received as
the event data for future use.

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