net: ipa: define SMEM memory region for IPA

Arrange to use an item from SMEM memory for IPA.  SMEM item number
497 is designated to be used by the IPA.  Specify the item ID and
size of the region in platform configuration data.  Allocate and get
a pointer to this region from ipa_mem_init().  The memory must be
mapped for access through an SMMU.

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