[top_earlgrey, dv] Initialize OTBN IMEM and DMEM to all 0
[top_earlgrey, dv] Initialize OTBN IMEM and DMEM to all 0

For some chip-level tests such as otbn_mem_scramble selected memories
may need to be initialized to enable these tests reading the memories
before writing. Obviously, this will then trigger ECC integrity errors,
but this is what these tests usually want to observe anyways. Therefore,
this commit initializes OTBN IMEM and DMEM with all 0.

Without initializing these memories, software running on Ibex and OTBN
has to completely write these memories which is rather slow. Otherwise,
unknown values will propagate into the system e.g. through the bus and
trigger various assertion failures thereby aborting the test.

This is related to lowRISC/OpenTitan#14196.

Signed-off-by: Pirmin Vogel <386a7fc8e7338ed7899b4d9883c0ebec5de81366@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan