net: ipa: kill IPA_MEM_UC_OFFSET

The microcontroller shared memory area is at the beginning of the
IPA resident memory.  IPA_MEM_UC_OFFSET was defined as the offset
within that region where it's found, but it's 0, and it's never
actually used.  Just get rid of the definition, and move some of the
description it had to be above the definition of the ipa_uc_mem_area
structure.

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