net: fec: add clk_prepare/clk_unprepare

It's for migrating to generic clk framework API.

Signed-off-by: Richard Zhao <richard.zhao@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: David S. Miller <davem@davemloft.net>
Acked-by: Marek Vasut <marek.vasut@gmail.com>