[otbn,doc] Document the bus-accessible part of DMEM
[otbn,doc] Document the bus-accessible part of DMEM

This doesn't yet change the implementation, nor does it change DV or
tooling code.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

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