Make the offset from the beginning of the "reg" property be from the
starting bus number, rather than zero.  Hoist the invariant size
calculation out of the mapping for loop.

Update host-generic-pci.txt to clarify the semantics of the "reg" property
with respect to non-zero starting bus numbers.
Signed-off-by: NDavid Daney <david.daney@cavium.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NArnd Bergmann <arnd@arndb.de>
Acked-by: NRob Herring <robh@kernel.org>