net: ipa: define IPA_MEM_END_MARKER

Define a new pseudo memory region identifer that specifies the
offset at the end of IPA resident memory.  Use it instead of
IPA_MEM_UC_EVENT_RING in places where the size of that region was
defined to be 0.

The size of the IPA_MEM_END_MARKER pseudo region must be zero.

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