ARM: dts: mxs: add the "clock-names" for gpmi-nand

The current DT nodes for mx23/mx28 miss the `clocks-names` item for gpmi-nand.
So the gpmi-nand driver could not find the proper clock.

This patch fixes this issue.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>