ARM: at91/gpio: non-DT builds do not have gpio_chip.of_node field

Protect build failure in case of non-DT configuration: the
gpio_chip structure does not have a of_node field in case of
!CONFIG_OF_GPIO.

Keep this in a separate patch as it can be reverted if the
field is added for both DT/non-DT cases.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>