net/smsc911x: Provide common clock functionality

Some platforms provide clocks which require enabling before the
SMSC911x chip will power on. This patch uses the new common clk
framework to do just that. If no clock is provided, it will just
be ignored and the driver will continue to assume that no clock
is required for the chip to run successfully.

Acked-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>