pci: Move PCI device configuration check into a separate weak function

This patch moves the check, if a device should be skipped in PCI PNP
configuration into the function pci_skip_dev(). This function is defined
as weak so that it can be overwritten by a platform specific one if
needed. The check if the device should get printed in the PCI summary upon
bootup (when CONFIG_PCI_SCAN_SHOW is defined) is moved to the function
pci_print_dev() which is also defined as weak too.

Signed-off-by: Stefan Roese <sr@denx.de>