Update cachgrind instructions to match new default values for cache profiling

[Tropicao]

Valgrind v3.21.0 has changed the default value of --cache-sim from yes to no. This change has been introduced because "The cache simulation is old and unlikely to match any real modern machine".
Based on the official documentation, it means that starting from this version, only instruction cache reads will be accounted when running cachegrind. Current labs are still not affected while we stay on an older version of valgrind, but at some point we will update valgrind and lab won't work anymore (it currently relies on data cache write misses). Moreover, trainees which will likely use updated version of valgrind will not be able to apply topics discovered during the labs.

We have to check for the following points:

• how accurate is cachegrind cache simulation regarding our lab platform (STM32MP157DK1) ?
• if it is accurate enough, we should add instructions to properly re-enable cache simulation: -cache-sim=yes
• if it is not accurate enough, we should either:
  • change the studied metric ?
  • change the tool ?

[Tropicao]

This issue has already been dealt with when upgrading Bootlin's fork of buildroot, the corresponding parameter is already part of the slides and labs instruction