soc: microchip: add mpfs gpio interrupt mux driver

soc: microchip: add mpfs gpio interrupt mux driver

On PolarFire SoC there are more GPIO interrupts than there are interrupt
lines available on the PLIC, and a runtime configurable mux is used to
decide which interrupts are assigned direct connections to the PLIC &
which are relegated to sharing a line.

Add a driver so that Linux can set the mux based on the interrupt
mapping in the devicetree.

Reviewed-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>