[ARM] pxa/cm-x270: add libertas device registration

v2 changes:
- use gpio_to_irq instead of IRQ_GPIO
- check gpio_direction_output return value to be on the safe side :)

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>