dm: pci: Add functions to emulate 8- and 16-bit access

Provide a few functions to support using 32-bit access to emulate 8- and
16-bit access.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>