spi: xilinx: make IRQs optional

Merge series from Abdurrahman Hussain <abdurrahman@nexthop.ai>:

Additionally, make interrupts optional to allow the driver to fall back
to its existing polling mode on systems where interrupts are either missing
or broken.

diff --cc MAINTAINERS
Simple merge

diff --cc drivers/spi/spi-xilinx.c
Simple merge