gpio: mxc: Use helper function devm_clk_get_optional_enabled()

Since commit 7ef9651e ("clk: Provide new devm_clk helpers for
prepared and enabled clocks"), devm_clk_get_optional() and
clk_prepare_enable() can now be replaced by
devm_clk_get_optional_enabled() when the 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.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>