mtd: gpmi-lib: convert to clk_prepare/clk_unprepare

The patch converts gpmi nand driver to clk_prepare/clk_unprepare by
using helper functions clk_prepare_enable/clk_disable_unprepare.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Huang Shijie <b32955@freescale.com>
Cc: Artem Bityutskiy <artem.bityutskiy@intel.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>