[TF-M] irqs "source" field

I think the PSA FF current definition of the "irqs" "source" attribute is needlessly opaque, confusing, and limiting.

Why can't there simply be two attributes "irq_number" and "irq_name".

"irq_number" corresponds to the CMSIS XYZ_IRQn convention in terms of its interpretation. It can be either a literal number or a manifest constant.

"irq_name" is the symbolic name of the ISR as it appears in the user's vector table.

That way there is no confusion regarding how they will be used.

Alan