target/riscv: fix addressing in `dm_read`/`dm_wirte`

There was an error in `dm_read`/`dm_write`: DMI address was checked
against DM registers disregarding DM base address.

To solve the issue `dmi_address()` function was introduced.

Change-Id: Ia3be619417b5f5b53db5dfe302db05170d6787c9
Signed-off-by: Evgeniy Naydanov <evgeniy.naydanov@syntacore.com>