mtd: nand: fsmc: use devm_clk_get()

This commit switches the fsmc_nand driver from clk_get() to
devm_clk_get(), which saves a few clk_put().

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>