dt: disable self-tests for !OF_IRQ

Fix OF selftest compile on sparc which does not enable OF_IRQ.

drivers/of/selftest.c:177: undefined reference to `of_irq_parse_one'
drivers/of/selftest.c:197: undefined reference to `of_irq_parse_one'
drivers/of/selftest.c:248: undefined reference to `of_irq_parse_one'

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@linaro.org>