of: irq: use of_property_read_u32()

The DT interrupt parsing code predates of_property_read_u32(), so it has to
basically open-code it. Using the modern DT API saves several LoCs and also
adds some prop sanity checks as a bonus...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Rob Herring <robh@kernel.org>