phy: qcom-qmp: Use proper PWRDOWN offset for sm8150 USB

The register map for SM8150 QMP USB SSPHY has moved
QPHY_POWER_DOWN_CONTROL to a different offset.  Allow for
an offset in the register table to override default value
if it is a DP capable PHY.

Signed-off-by: Wesley Cheng <wcheng@codeaurora.org>
Reviewed-by: Manu Gautam <mgautam@codeaurora.org>
Link: https://lore.kernel.org/r/1588636467-23409-5-git-send-email-wcheng@codeaurora.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>