mtd: rawnand: mpc5121: Use helper function devm_clk_get_enabled()

Since commit 7ef9651e ("clk: Provide new devm_clk helpers for prepared
and enabled clocks"), devm_clk_get() and clk_prepare_enable() can now be
replaced by devm_clk_get_enabled() when driver enables (and possibly
prepares) the clocks for the whole lifetime of the device. Moreover, it is
no longer necessary to unprepare and disable the clocks explicitly.

Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Li Zetao <lizetao1@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230821031737.1973183-7-lizetao1@huawei.com