[otbn] Explicitly ignore the bottom bits of dmem_addr_core
[otbn] Explicitly ignore the bottom bits of dmem_addr_core

This signal comes out of the core with width DmemAddrWidth, but the
bottom bits are always zero. We were already ignoring the bottom bits
of imem_addr_core: do the same for dmem.

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

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