x86: Add a simple interrupt script to the README

It is a bit tedious to figure out the interrupt configuration for a new
x86 platform. Add a script which can do this, based on the output of
'pci long'. This may be helpful in some cases.

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