mtd: rawnand: marvell: Rename ->ecc_clk into ->core_clk

The core clock field was badly named ->ecc_clk which might lead to some
confusion. Rename it ->core_clk.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>