arch: irq: multilevel: the value return from irq_parent_level_3() is wrong

[ycsin]

Describe the bug

When Zephyr encodes an IRQ to the second and the third level, the value of the IRQ is incremented by one, as zero means no IRQs present in that level.

So when we want to get the parent IRQ of level 3 with irq_parent_level_3(), the value in the level 2 should be restored by minus 1.

To Reproduce

1. Run the tests fixed/added in irq: multilevel: fixes and optimizations  #78845 with v3.7-branch
2. The test should fail

Expected behavior
irq_parent_level_3() should return the correct IRQ and the test should pass

Impact
Applications/drivers that depend on irq_parent_level_3() will get the wrong IRQ value.

Context
This can be closed by backporting the first commit (30aa346) of #78845

[ycsin]

This issue is for the backport, which I havent create yet