lib: Fix ipi type update

IPIs are updated in scratch space by source hart.
However, different harts or same hart may want to
send different IPIs before previous IPI was read
by the target hart. Currently, previous IPI type
is overwritten in that case.

Use atomic bit set/clear operations to update IPIs.

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