MIPS: OCTEON: irq: Fix octeon_irq_force_ciu_mapping()

MIPS: OCTEON: irq: Fix octeon_irq_force_ciu_mapping()

For irq_domain_associate() to work the virq descriptor has to be
pre-allocated in advance. Otherwise the following happens:

WARNING: CPU: 0 PID: 0 at .../kernel/irq/irqdomain.c:527 irq_domain_associate+0x298/0x2e8
error: virq128 is not allocated
Modules linked in:
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.19.78-... #1
        ...
Call Trace:
[<ffffffff801344c4>] show_stack+0x9c/0x130
[<ffffffff80769550>] dump_stack+0x90/0xd0
[<ffffffff801576d0>] __warn+0x118/0x130
[<ffffffff80157734>] warn_slowpath_fmt+0x4c/0x70
[<ffffffff801b83c0>] irq_domain_associate+0x298/0x2e8
[<ffffffff80a43bb8>] octeon_irq_init_ciu+0x4c8/0x53c
[<ffffffff80a76cbc>] of_irq_init+0x1e0/0x388
[<ffffffff80a452cc>] init_IRQ+0x4c/0xf4
[<ffffffff80a3cc00>] start_kernel+0x404/0x698

Use irq_alloc_desc_at() to avoid the above problem.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>