irqchip/jcore-aic: Fix missing allocation of IRQ descriptors

The initialization function for the J-Core AIC aic_irq_of_init() is
currently missing the call to irq_alloc_descs() which allocates and
initializes all the IRQ descriptors. Add missing function call and
return the error code from irq_alloc_descs() in case the allocation
fails.

Fixes: 981b58f6 ("irqchip/jcore-aic: Add J-Core AIC driver")
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Tested-by: Rob Landley <rob@landley.net>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230510163343.43090-1-glaubitz@physik.fu-berlin.de