net: stmmac: add platform level clocks management

This patch intends to add platform level clocks management. Some
platforms may have their own special clocks, they also need to be
managed dynamically. If you want to manage such clocks, please implement
clks_config callback.

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>