media: qcom: camss: csiphy: Add an init callback to CSI PHY devices

Add a nop init callback to CSIPHY devices, this callback is used to add
some HW register offset and register configuration for specific platform,
then different platform can reuse the same CSIPHY driver. Later changes
will enumerate with enabling code.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Depeng Shao <quic_depengs@quicinc.com>
Reviewed-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>