gpio/langwell: convert to use irq_domain

irq_domain already provides a facility to translate from hardware IRQ
numbers to Linux IRQ numbers so use that instead of open-coding the logic
in the driver.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>