dm: serial: Convert ns16550 driver to use driver model PCI API

dm: serial: Convert ns16550 driver to use driver model PCI API

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>