sh: intc: Drop the use of irq_create_identity_mapping()

Instead of playing games with using irq_create_identity_mapping()
and irq_domain_associate(), drop the use of the former and only
use the latter, together with the allocation of the irq_desc
as needed.

It doesn't make the code less awful, but at least the intent
is clearer.

Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Zyngier <maz@kernel.org>