net: ipa: store memory region id in descriptor

Store the memory region ID in the memory descriptor structure.  This
is a move toward *not* indexing the array by the ID, but for now we
must still specify those index values.  Define an explicitly
undefined region ID, value 0, so uninitialized entries in the array
won't use an otherwise valid ID.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>