x86: Add a x86_ prefix to the x86-specific PCI functions

These functions currently use a generic name, but they are for x86 only.
This may introduce confusion and prevents U-Boot from using these names
more widely.

In fact it should be possible to remove these at some point and use
generic functions, but for now, rename them.

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