stmmac: dwmac-mediatek: re-arrange clock setting

The rmii_internal clock is needed only when PHY
interface is RMII, and reference clock is from MAC.

Re-arrange the clock setting as following:
1. the optional "rmii_internal" is controlled by devm_clk_get(),
2. other clocks still be configured by devm_clk_bulk_get().

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>