clk: mxs: Fix the GPMI clock name

The correct name for the GPMI clock is 'gpmi-nand'.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>