In preparation for EVA support, we use a macro to build the
__csum_partial_copy_user main code so it can be shared across
multiple implementations. EVA uses the same code but it replaces
the load/store/prefetch instructions with the EVA specific ones
therefore using a macro avoids unnecessary code duplications.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>