mtd: mxc_nand: Adapt the clock name to the new clock framework

With the new i.mx clock framework the mxc_nand clock is registered as:

clk_register_clkdev(clk[nfc_gate], NULL, "mxc_nand");0")

So we do not need to pass "nfc" string and can use NULL instead.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>