net: stmmac: dwmac-meson8b: make the clock configurations private

The common clock framework needs access to the "clock configuration"
structs during runtime.
However, only the common clock framework should access these. Ensure
this by moving the configuration structs out of struct meson8b_dwmac,
so only meson8b_init_rgmii_tx_clk() and the common clock framework know
about these configurations.

Suggested-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>