Use devm_clk_get_optional() to get optional clock

Cc: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>