drivers: lpc_gpio: Fix warning when interrupt is not defined
drivers: lpc_gpio: Fix warning when interrupt is not defined

Unused variable warnings were seen when the GPIO interrupt
property was not defined.

Signed-off-by: Mahesh Mahadevan <7f315ec3481825d543f8bec256ad86a4adc9dd8a@nxp.com>

finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr