dm: pci: Move pci_get_devfn() into a common file

Early in boot it is necessary to decode the PCI device/function values for
particular peripherals in the device tree or of-platdata. This is needed
in TPL where CONFIG_PCI is not defined.

To handle this, move pci_get_devfn() into a file that is built even when
CONFIG_PCI is not defined.

Also add a function for use by of-platdata, to convert a reg property to
a pci_dev_t.

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