mtd: mpc5121_nfc: cleanup clock API use

use devm_clk_get() for automatic put after device close, check for and
propagate errors when enabling clocks, need to prepare clocks before
they can get enabled, adjust error code paths to correctly balance
get/put and prepare/unprepare and enable/disable calls

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>