gpio-langwell: fix irq conflicts when DT is not used

When DT is not used IOAPIC does not register irq domain. As result
IOAPIC won't care about gpio-langwell's virq and may cause conflict if
an irq number is equal to gpio-langwell's virq mapped beforehand.

This patch tells gpio_langwell to not ignore irq_base if != 0 and
preferably use it to avoid the conflict.

If DT is used, irq_base can safely be 0.

Signed-off-by: David Cohen <david.a.cohen@intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>