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 <[e-mail]>
Cc: Artem Bityutskiy <[e-mail]>
Cc: [e-mail]
Signed-off-by: Gerhard Sittig <[e-mail]>
Signed-off-by: Anatolij Gustschin <[e-mail]>

drivers/mtd/nand/mpc5121_nfc.c

···
 	of_node_put(rootnode);
 
 	/* Enable NFC clock */
-	clk = devm_clk_get(dev, "nfc_clk");
+	clk = devm_clk_get(dev, "ipg");
 	if (IS_ERR(clk)) {
 		dev_err(dev, "Unable to acquire NFC clock!\n");
 		retval = PTR_ERR(clk);