pcie-xilinx: OF-PCIe numbers legacy interrupts from 1

Without this patch we can't use the 4th legacy PCIe interrupt.  This
is a workaround: there are only 4 legacy interrupts, but since they're
numbered starting from 1 instead of 0 this just adds a 5th interrupt.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Wesley W. Terpstra <wesley@sifive.com>