pci: Update documentation to make 'compatible' string optional

Reword the documentation to make it clear the compatible string is now
optional, yet still matching on it takes precedence over PCI IDs and
PCI classes.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>