drivers: gpio: xlnx_axi: fix polling mode

Fix compiler errors and warnings that are issued when compiling this
driver for use without the optional interrupt facilities.

When interrupts are not enabled for any instance of this device,
there's currently a compiler error in gpio_xlnx_axi_pin_interrupt_configure
due to the function's header being located within the ifdef-block for
interrupt-driven operation.

Fully encapsulate the interrupt-related functions
gpio_xlnx_axi_pin_interrupt_configure, gpio_xlnx_axi_manage_callback
and gpio_xlnx_axi_get_pending_int with ifdefs checking for interrupt
mode inorder to remove compiler warnings caused by those functions
being always present so far, while not being used in the API function
pointer table when all instances of this GPIO controller are operating
in polling mode. This circumstance causes a "declared, but not used"
compiler warning for each of those functions.

Signed-off-by: Immo Birnbaum <mail@birnbaum.immo>