MLBEDSW-6384: Updated weight buffering cycle calculation

- The npu cycles are not correct calculated when only
one weight buffer is used, since weights can not
be fetched in parallel.
- Added new calculation in the single buffer case.

Signed-off-by: Johan Alfven <johan.alfven@arm.com>
Change-Id: I8568912d11d137a298225ab77b8b3272613c76f6