REDUCE_SUM: Specify the accumulator type

For bf16_t data type, REDUCE_SUM is changed to use fp32_t
as the accumulator type to be consistent with CONV2D.
For other data types the accumulator type is the same
as in_out_t (and so no change).
Also correct the rank limit text.

Update reduction pseudo-code to be consistent with
REDUCE_SUM.

Change-Id: I9923066be7d1b7edb0efd9bcf3365b4af9501beb
Signed-off-by: Dominic Symes <dominic.symes@arm.com>