net: stmmac: dwmac-imx: add platform level clocks management for i.MX

Split clocks settings from init callback into clks_config callback,
which could support platform level clocks management.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>