of/irq: Fix bug in interrupt parsing refactor.

Commit 23616132, "of/irq: Refactor interrupt-map parsing" introduced
a bug. The irq parsing will fail for some nodes that don't have a reg
property. It is fixed by deferring the check for reg until it is
actually needed. Also adjust the testcase data to catch the bug.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Ming Lei <tom.leiming@gmail.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Cc: Rob Herring <rob.herring@calxeda.com>