iio: xilinx-xadc: Make IRQ optional

iio: xilinx-xadc: Make IRQ optional

In some setups the IRQ signal of the XADC might not be wired to the host
system. The driver currently requires that an interrupt is specified. Make
the interrupt optional so the driver can be used in such setups where the
interrupt is not connected.

Since both the internal triggers as well as events depend on the interrupt
being connected both are not available when the interrupt is not connected.

Buffered access is still supported even without an interrupt since an
external trigger can be used.

The IRQ is only optional when using the AXI interface, since the PCAP
interface needs the IRQ for reading and writing registers.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20211021125950.28707-1-lars@metafoo.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>