plat: clint: Reduce use of atomic operation on IPI registers

Currently, we aggresively use atomic operation on CLINT IPI
register. This patch simplify CLINT IPI APIs by reducing use
of atomic operations. In future, we will gradually increase
use of atomic operations for CLINT IPI APIs.

Signed-off-by: Anup Patel <anup.patel@wdc.com>