ravb: Use separate clock for gPTP

RZ/V2M has a separate gPTP reference clock that is used when the
AVB-DMAC Mode Register (CCC) gPTP Clock Select (CSEL) bits are
set to "01: High-speed peripheral bus clock".
Therefore, add a feature that allows this clock to be used for
gPTP.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>