pci: Match region flags using a mask

When converting addresses, apply a mask to the region flags during
lookup. This allows the caller to specify which flags are important and
which are not, for example to exclude system memory regions.

The behaviour of the function is changed such that they don't
preferentially search for a non-system memory region. However, system
memory regions are added after other regions in decode_regions() leading
to a similar outcome.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>