[sw, otbn] Skip zeroing of DMEM inside otbn_load_app()
[sw, otbn] Skip zeroing of DMEM inside otbn_load_app()

It is much more efficient to just perform a secure wipe of DMEM instead
of having Ibex clearing the entire DMEM to 0 (~1000 vs ~222000 clock
cycles). The secure wipe is also better from a security perspective as
it effectively randomly precharges the entire DMEM.

If software still wants to clear DMEM to 0, e.g. to ease SCA during
prototyping, it can do so by calling otbn_zero_data_memory() before
loading the app.

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

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