net: stmmac: dwmac-qcom-ethqos: add platform level clocks management

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

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>