ARM: PXA: Fix cplds irqdesc allocation when using legacy mode

The Mainstone PXA platform uses CONFIG_SPARSE_IRQ, and thus we
cannot rely on the irq descriptors to be readilly allocated
before creating the irqdomain in legacy mode. The kernel then
complains loudly about not being able to associate the interrupt
in the domain -- can't blame it.

Fix it by allocating the irqdescs upfront in the legacy case.

Fixes: b68761da

 ("ARM: PXA: Kill use of irq_create_strict_mappings()")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210426223942.GA213931@roeck-us.net