irqchip/gicv3: Handle resource request failure consistently

Due to a silly oversight on my part, making the simple switch to
of_io_request_and_map() in the DT path inadvertently introduced
divergent behaviour, whereby failng to request an iomem region now
becomes fatal for DT, vs. being silently ignored for ACPI.

Refactor a bit harder, so that request errors are non-fatal in both
paths as intended, but also consistently reported as well.

Reported-by: Matt Ranostay <mranostay@ti.com>
Fixes: 2b2cd74a ("irqchip/gic-v3: Claim iomem resources")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/5f2b57a0131f3082fae9d3002d360bf784ccb092.1655387206.git.robin.murphy@arm.com