dpaa2-eth: Provide direct access to 1588 one step register

commit b444194f27977a284e014ba0d531e02101d17413 from
https://source.codeaurora.org/external/imx/linux-imx

The DPAA2 MAC supports 1588 one step timestamping.
If this option is enabled then for each transmitted PTP event packet,
the 1588 SINGLE_STEP register is accessed to modify the following fields:

-offset of the correction field inside the PTP packet
-UDP checksum update bit,  in case the PTP event packet has
 UDP encapsulation

These values can change any time, because there may be multiple
PTP clients connected, that receive various 1588 frame types:
- L2 only frame
- UDP / Ipv4
- UDP / Ipv6
- other

The current implementation uses dpni_set_single_step_cfg to update the
SINLGE_STEP register.
Using an MC command  on the Tx datapath for each transmitted 1588 message,
introduces high delays, leading to low throughput and consequently to a
small number of supported PTP clients. Besides these, the nanosecond
correction field from the PTP packet will contain the high delay from the
driver which together with the originTimestamp will render timestamp
values that are unacceptable in a GM clock implementation.

This patch replaces the dpni_set_single_step_cfg function call on the
Tx datapath for 1588 messages (when one step timestamping is enabled) with
a callback that either implements direct access to the SINGLE_STEP
register, eliminating the overhead caused by the MC command that will need
to be dispatched by the MC firmware through the MC command portal
interface or falls back to the dpni_set_single_step_cfg in case the MC
version does not have support for returning the single step register
base address.

In other words all the delay introduced by dpni_set_single_step_cfg
function will be eliminated (if MC version has support for returning the
base address of the single step register), improving the egress driver
performance for PTP packets when single step timestamping is enabled.

Signed-off-by: Radu Bulie <radu-andrei.bulie@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>