[otbn] default increment dmem pointer
[otbn] default increment dmem pointer

for dmem access operations with two pointer increments
(reg pointer and dmem) pointer only one increment can be
handled in the load/store instruction. The other increment
is handeled in a subsequent `addi`. This adjusts the increments
such that always the dmem pointer is incremented in the load/store
and the reg pointer in the `addi`. This has the advantage that an
immediate of `1` can be used for the `addi` instruction regardless
of the type of addressing used for dmem.

Signed-off-by: Felix Miller <43d2a12732a784b305469416c0ba5ce1795e752a@gi-de.com>

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