irqchip/sifive-plic: Fix T-HEAD PLIC edge trigger handling

The T-HEAD PLIC ignores additional edges seen while an edge-triggered
interrupt is being handled. Because of this behavior, the driver needs
to complete edge-triggered interrupts in the .irq_ack callback before
handling them, instead of in the .irq_eoi callback afterward. Otherwise,
it could miss some interrupts.

Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220630100241.35233-5-samuel@sholland.org