Since the simple irqdomain will fall back to a linear domain
if the first_irq provided is <= 0, just use this, just make
sure the first_irq is negative in the device tree case.

Cc: Rob Herring <rob.herring@calxeda.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>