of_pci: use of_property_read_u32()

of_get_pci_domain_nr() somehow didn't use of_property_read_u32() though it
was long available, basically open-coding it. Using the modern DT API saves
several LoCs/bytes and also adds some prop sanity checks as a bonus...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Rob Herring <robh@kernel.org>