mtd: mpc5121_nfc: adjust for OF based clock lookup

after device tree based clock lookup became available, the NAND
flash driver need no longer use the previous global "nfc_clk" name,
but should use the "ipg" clock name specific to the OF node

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>