use devm_get_clk() for automatic put upon device release, check for and
propagate errors when enabling clocks, must prepare clocks before they
can get enabled, unprepare after disable

need to use the _parent_ of the platform device for clock lookup, since
this one is associated with the respective device tree node; this change
remains neutral as long as a "globally" provided "usb%d_clk" item gets
provided by either the PPC_CLOCK implementation or clkdev_register'ed
aliases, using the correct devide and thus referencing the right DT node
becomes essential when clock lookup will become based on device tree
when common clock support will get introduced
Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>