pci: Add helper for implementing memory-mapped config space accesses

This sort of pattern for implementing memory-mapped PCI config space
accesses appears in U-Boot twice already, and a third user is coming up.
So add helper functions to avoid code duplication, similar to how Linux
has pci_generic_config_write and pci_generic_config_read.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>