x86: pci: Add a function to clear and set PCI config regs

At present the x86 pre-DM equivalent of pci_bus_clrset_config32() does not
exist. Add it to simplify PCI init code on x86.

Also add the missing functions to this header.

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