keystone2: add wfi in to the core_spin loop

When core A turning of core B, via tetris DPSC it places the core
B DPSC into transitional state. The core B has to execute wfi instruction
to move its DPSC to the OFF state. This patch add such instruction.

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>