of: Add testcases for interrupt parsing

This patch extends the DT selftest code with some test cases for the
interrupt parsing functions.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

diff --cc arch/arm/boot/dts/testcases/tests-interrupts.dtsi
Simple merge

diff --cc arch/arm/boot/dts/testcases/tests.dtsi
Simple merge

diff --cc drivers/of/selftest.c
Simple merge