irqchip/xilinx: Switch to GENERIC_IRQ_MULTI_HANDLER

Register the Xilinx driver as the root interrupt controller using
the GENERIC_IRQ_MULTI_HANDLER API, instead of the arch-specific hack.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Stefan Asserhall <stefan.asserhall@xilinx.com>
[maz: repainted commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/e6c6595a81f662bf839cee3109d0fa58a596ea47.1646380284.git.michal.simek@xilinx.com