PCI: mediatek: Get optional clocks with devm_clk_get_optional()

Update the driver to use devm_clk_get_optional() to claim
optional clocks instead of devm_clk_get().

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>
Acked-by: Honghui Zhang <honghui.zhang@mediatek.com>
Cc: Ryder Lee <ryder.lee@mediatek.com>
Cc: Honghui Zhang <honghui.zhang@mediatek.com>