drivers: pci: Fix dm_pci_map_bar() to support 64b BARs

This enables 64b BARs if CONFIG_SYS_PCI_64BIT is enabled.

Reviewed-by: Philip Oberfichtner <pro@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Moritz Fischer <moritzf@google.com>