x86: Tidy up the PIRQ routing code a little

This code could use a little tightening up. There is some repetition and
an odd use of fdtdec_get_int_array().

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>