x86: irq: Parse number of PIRQ links from device tree

The "intel,pirq-link" property in Intel IRQ router's dt bindings
has two cells, where the second one represents the number of PIRQ
links on the platform. However current driver does not parse this
information from device tree. This adds the codes to do the parse
and save it for future use.

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