of: fix PCI bus match for PCIe slots

On IBM pseries systems the device_type device-tree property of a PCIe
bridge contains the string "pciex". The of_bus_pci_match() function was
looking only for "pci" on this property, so in such cases the bus
matching code was falling back to the default bus, causing problems on
functions that should be using "assigned-addresses" for region address
translation. This patch fixes the problem by also looking for "pciex" on
the PCI bus match function.

v2: added comment

Signed-off-by: Kleber Sacilotto de Souza <klebers@linux.vnet.ibm.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Cc: stable@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>