pci: serial: Support reading PCI-register size with base

The PCI helpers read only the base address for a PCI region. In some cases
the size is needed as well, e.g. to pass along to a driver which needs to
know the size of its register area.

Update the functions to allow the size to be returned. For serial, record
the information and provided it with the serial_info() call.

A limitation still exists in that the size is not available when OF_LIVE
is enabled, so take account of that in the tests.

Signed-off-by: Simon Glass <sjg@chromium.org>