net: ravb: Move PTP initialization in the driver's ndo_open API for ccc_gac platorms

The initialization sequence for PTP is the same for platforms with ccc_gac
and gptp (according to "Figure 50.71 Flow of gPTP Initialization (Normal,
Common to All Modes)" of the R-Car Series, 3rd generation hardware
manual and "Figure 37A.53 Flow of gPTP Initialization (Normal, Common to
All Modes)" of the RZ/G Series hardware manual).

As some IP variants switch to reset mode (and thus the registers content is
lost) when setting clocks (due to module standby functionality) to be able
to implement runtime PM, move the PTP initialization to the driver's
ndo_open API.

This commit prepares the code for the addition of runtime PM.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>