drivers: uart: uart_cmsdk_apb: Fix compile warning.
drivers: uart: uart_cmsdk_apb: Fix compile warning.

If we are building the driver with CONFIG_UART_INTERRUPT_DRIVEN=n
we need to ifdef around the decleration of uart_cmsdk_apb_isr()
or we'll get a compiler warning.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

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