of: irq: use of_property_read_bool() for "interrupt-controller" prop

The "interrupt-controller" property is boolean, i.e. has no value.  The DT
interrupt parsing code predates of_property_read_bool(), so it uses either
of_get_property() or of_find_property() -- the former isn't quite correct
for the boolean props (but works somehow). Use the modern boolean prop API
instead.

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