fs_enet: cleanup clock API use

make the Freescale ethernet driver get, prepare and enable the FEC clock
during probe(); disable and unprepare the clock upon remove(), put is
done by the devm approach; hold a reference to the clock over the period
of use.

clock lookup is non-fatal as not all platforms provide clock specs in
their device tree; failure to enable specified clocks is fatal.

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>