Use the driver model version of the function to find the BAR. This updates
the fdtdec function, of which ns16550 is the only user.

The fdtdec_get_pci_bdf() function is dropped for several reasons:
- with driver model we should use 'struct udevice *' rather than passing the
   device tree offset explicitly
- there are no other users in the tree
- the function parses for information which is already available in the PCI
device structure (specifically struct pci_child_platdata which is available
at dev_get_parent_platdata(dev)

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