ravb: Factorise ravb_dmac_init function

The DMAC IP on the R-Car AVB module has different initialization
parameters for RCR, TGC, TCCR, RIC0, RIC2, and TIC compared to
DMAC IP on the RZ/G2L Gigabit Ethernet module. Factorise the
ravb_dmac_init function to support the later SoC.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>