irqchip/al-fic: Support per-child affinity via interrupt redirection

The FIC multiplexes up to 32 sources into one parent GIC SPI, and a
child's handler runs in the parent's context, so a child has no affinity
control of its own. irq_set_affinity() therefore fails with -EINVAL, and
drivers that treat that as fatal cannot probe when their interrupt
arrives through a FIC group. The parent routing cannot be changed on
behalf of one child either, because several FIC groups may share one SPI
and all of them would move with it.

Use the interrupt redirection infrastructure. Dispatch children with
generic_handle_demux_domain_irq() and set irq_set_affinity to
irq_chip_redirect_set_affinity(), so a child whose affinity excludes the
CPU taking the parent interrupt runs on its target CPU in IRQ work
context. The interrupt is still taken on the parent's CPU; only the
child's handler body moves.

Redirection defers the whole flow handler, so the cause bit is cleared in
al_fic_irq_pre_redirect(), which runs in parent context on both the
inline and the redirected path. A level configured group is masked there
too, because its cause bit re-latches while the input stays asserted; the
deferred handle_level_irq() masks again and unmasks when the handler is
done. An edge configured group is only acked, since handle_edge_irq() has
no unconditional unmask. irq_ack is a noop because irq_pre_redirect runs
on every dispatch and the ack must not happen twice.

Signed-off-by: Eliav Farber <farbere@amazon.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Radu Rendec <radu@rendec.net>
Link: https://patch.msgid.link/20261011050449.41903-10-farbere@amazon.com

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