Some USB subsystems that utlize the M31 eUSB2 PHY doesn't require a clkref.
One example is the Glymur platform, which contains 4 USB controllers that
all utilize the M31 eUSB2 PHY.  However, PHYs associated to the primary
and secondary controllers do not require a clkref resource.  Due to this,
mark this as an optional clock.

Signed-off-by: Wesley Cheng <wesley.cheng@oss.qualcomm.com>
---
 drivers/phy/qualcomm/phy-qcom-m31-eusb2.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/phy/qualcomm/phy-qcom-m31-eusb2.c b/drivers/phy/qualcomm/phy-qcom-m31-eusb2.c
index fbf5e999ca7a..f4355d38aad9 100644
--- a/drivers/phy/qualcomm/phy-qcom-m31-eusb2.c
+++ b/drivers/phy/qualcomm/phy-qcom-m31-eusb2.c
@@ -267,7 +267,7 @@ static int m31eusb2_phy_probe(struct platform_device *pdev)
 	if (IS_ERR(phy->reset))
 		return PTR_ERR(phy->reset);
 
-	phy->clk = devm_clk_get(dev, NULL);
+	phy->clk = devm_clk_get_optional(dev, NULL);
 	if (IS_ERR(phy->clk))
 		return dev_err_probe(dev, PTR_ERR(phy->clk),
 				     "failed to get clk\n");