watchdog: at91sam9_wdt: Use the devm_clk_get_enabled() helper function

The devm_clk_get_enabled() helper:
    - calls devm_clk_get()
    - calls clk_prepare_enable() and registers what is needed in order to
      call clk_disable_unprepare() when needed, as a managed resource.

This simplifies the code.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20230824135514.2661364-2-ruanjinjie@huawei.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>