dm: pci: Add a function to read a PCI BAR

dm: pci: Add a function to read a PCI BAR

At present PCI address transaction is not supported so drivers must
manually read the correct BAR after reading the device tree info. The
ns16550 has a suitable implementation, so move this code into the core
DM support.

Note that there is no live-tree equivalent at present.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: correct the unclear comments in test.dts]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>