irqdomain: Beef up debugfs output

This patch increases the amount of output produced by the
irq_domain_mapping debugfs file by first listing all of the registered
irq domains at the beginning of the output, and then by including all
mapped IRQs in the output, not just the active ones. It is very useful
when debugging irqdomain issues to be able to see the entire list of
mapped irqs, not just the ones that happen to be connected to devices.

Signed-off-by: Grant Likely <grant.likely@linaro.org>