powerpc/of/pci: Rewrite pci_parse_of_flags

The existing code uses bunch of hardcoded values from the PCI Bus
Binding to IEEE Std 1275 spec; and it does so in quite non-obvious
way.

This defines fields from the cell#0 of the "reg" property of a PCI
device and uses them for parsing.

This should cause no behavioral change.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[mpe: Unsplit some 80/81 char lines, space the code with some newlines]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190829084417.71873-1-aik@ozlabs.ru