arch: arc: secure world only check secure interrupt

* in arc secureshield interrupts can be configured
  as secure or normal
* in sw design, high interrupt priorites are allocated to
  secure world, low priorities are allocated to normal world.
* secure interrupt > secure thread > normal interrupt > normal
  thead

So, here secure world/firmware only checks secure interrupt
priorities

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>