mailbox: sprd: Use devm_clk_get_enabled() helpers

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

This simplifies the code and avoids the calls to clk_disable_unprepare().

Due to clk only used in probe, not in suspend\resume, this pointer can
remove from sprd_mbox_priv to save a little memory.

Signed-off-by: Huan Yang <link@vivo.com>
Reviewed-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>