use irq pin as the pin is asserted untill we clear it
This will allow to do not poll on i2c which slow down barebox

If no irq_pin is provided fall back on i2c polling

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>