Revert "irqchip/xilinx: Enable generic irq multi handler"

This reverts commit a0789993, which
breaks a number of PPC platforms.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/44b64be7-9240-fd52-af90-e0245220f38b@xilinx.com