irqchip/versatile-fpga: Fix PCI IRQ mapping on Versatile PB

This patch is specifically for PCI support on the Versatile PB board using
a DT. Currently, the dynamic IRQ mapping is broken when using DTs. For
example, on QEMU, the SCSI driver is unable to request the IRQ. To fix
this issue, this patch replaces the current dynamic mechanism with a
static value as is done in the non-DT case.

Signed-off-by: Guillaume Delbergue <guillaume.delbergue@greensocs.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: stable@vger.kernel.org