dpni_get_single_step_cfg is an MC firmware command used for
retrieving the contents of SINGLE_STEP 1588 register available
in a DPMAC.

This patch adds a new version of this command that returns as an extra
argument the physical base address of the aforementioned register.
The address will be used to directly modify the contents of the
SINGLE_STEP register instead of invoking the MC command
dpni_set_single_step_cgf. The former approach introduced huge delays on
the TX datapath when one step PTP events were transmitted. This led to low
throughput and high latencies observed in the PTP correction field.
Signed-off-by: Radu Bulie <radu-andrei.bulie@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>