x86: irq: Support discrete PIRQ routing registers via device tree

Currently both pirq_reg_to_linkno() and pirq_linkno_to_reg() assume
consecutive PIRQ routing control registers. But this is not always
the case on some platforms. Introduce a new device tree property
intel,pirq-regmap to describe how the PIRQ routing register offset
is mapped to the link number and adjust the irq router driver to
utilize the mapping.

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