irqchip/gic-v3: Fix selection of partition domain for EPPIs

commit 5f51f803 ("irqchip/gic-v3: Add EPPI range support") added
GIC_IRQ_TYPE_PARTITION support for EPPI to gic_irq_domain_translate(),
and commit 52085d3f ("irqchip/gic-v3: Dynamically allocate PPI
partition descriptors") made the gic_data.ppi_descs array big enough for
EPPI, but neither gic_irq_domain_select() nor partition_domain_translate()
were updated.

This means partitions are created by partition_create_desc() for the
EPPI range, but can't be registered as they will always match the root
domain and map to the summary interrupt.

Update gic_irq_domain_select() to match PPI and EPPI. The fwspec for
PPI and EPPI both start from 0. Use gic_irq_domain_translate() to find
the hwirq from the fwspec, then convert this to a ppi index.

Reported-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210729172748.28841-3-james.morse@arm.com