net: stmmac: dwmac-meson8b: use clk_parent_data for clock registration

Simplify meson8b_init_rgmii_tx_clk() by using struct clk_parent_data to
initialize the clock parents. No functional changes intended.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>