lib: sbi: sse: rename sse_hart_unlock() to sse_enabled_event_unlock()

There was a naming incoherency between enabled events list lock/unlock.
Rename sse_hart_unlock() to sse_enabled_event_unlock() to be coherent
and reword comments above lock()/unlock() functions.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>