lib: Use a fifo to keep track of sfence related IPIs.

Currently, there is no provision for tracking multiple IPIs sent
to a single hart at the same time by different harts.

Use a fifo manage the outstanding requests. While dequeueing, read all
the entries once, because we have only 1 bit to track the type of IPI.
Once the queue is full, busy wait until the there is space available in
queue. This is not the most elegant approach. It should be changed in
favor of a wakeup event once available in opensbi.

Signed-off-by: Atish Patra <atish.patra@wdc.com>