net: stmmac: Add platform level debug register dump feature

dwmac-qcom-ethqos currently exposes a mechanism to dump rgmii registers
after the 'stmmac_dvr_probe()' returns. However with commit
5ec55823 ("net: stmmac: add clocks management for gmac driver"),
we now let 'pm_runtime_put()' disable the clocks before returning from
'stmmac_dvr_probe()'.

This causes a crash when 'rgmii_dump()' register dumps are enabled,
as the clocks are already off.

Since other dwmac drivers (possible future users as well) might
require a similar register dump feature, introduce a platform level
callback to allow the same.

This fixes the crash noticed while enabling rgmii_dump() dumps in
dwmac-qcom-ethqos driver as well. It also allows future changes
to keep a invoking the register dump callback from the correct
place inside 'stmmac_dvr_probe()'.

Fixes: 5ec55823 ("net: stmmac: add clocks management for gmac driver")
Cc: Joakim Zhang <qiangqing.zhang@nxp.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>