samples: crypto: Fix mbedtls benchmark time calculations
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 <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

ldts/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,explora26/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,explora26/zephyr,explora26/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,explora26/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr