irqchip/xilinx: Enable generic irq multi handler

Register default arch handler via driver instead of directly pointing to
xilinx intc controller. This patch makes architecture code more generic.

Driver calls generic domain specific irq handler which does the most of
things self. Also get rid of concurrent_irq counting which hasn't been
exported anywhere.
Based on this loop was also optimized by using do/while loop instead of
goto loop.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Stefan Asserhall <stefan.asserhall@xilinx.com>
Link: https://lore.kernel.org/r/20200317125600.15913-4-mubin.usman.sayyed@xilinx.com

arch/microblaze/include/asm/irq.h

@@ -14,7 +14,4 @@
 struct pt_regs;
 extern void do_IRQ(struct pt_regs *regs);
 
-/* should be defined in each interrupt controller driver */
-extern unsigned int xintc_get_irq(void);
-
 #endif /* _ASM_MICROBLAZE_IRQ_H */