samples: crypto: Fix mbedtls benchmark time calculations

mbedTLS benchmark used 32-bit arithmetics for time calculations
(nanosecond resolution), which could overflow on slower platforms
on more time-consuming benchmark tests. In result, the benchmark
could give incorrect timing information. Using 64-bit arithmetics
prevents this issue.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>